Cubic chunks + no Y limit (foundation for LOD)

Chunks are now isotropic 16×16×16 cubes stacked in all three axes,
keyed by IVec3(cx, cy, cz). The world has no Y ceiling and no Y
floor — set_block lazily creates a chunk if one doesn't exist, so
building 1000 blocks straight up just creates 60-ish empty chunks
above the surface on demand.

world.rs:
  - CHUNK_HEIGHT removed. Single CHUNK_SIZE (=16) for all three axes.
  - Chunk allocates 16³ = 4096 blocks.
  - Chunk::get/set bound-check 0..CHUNK_SIZE on every axis.
  - block_to_chunk returns full 3D (cx, cy, cz) + (lx, ly, lz).
  - get_block has no Y bound — outside any chunk = Air (sky).
  - set_block: chunks.entry(c).or_insert_with(...) creates on demand.
  - HeightMap is keyed by IVec2(cx, cz) column. from_world_column scans
    every existing cubic chunk in that column top-down for the topmost
    solid block. Heightmap invalidation in set_block drops the column,
    not just the single chunk.
  - column_top_y, World::heightmap signature both take (cx, cz) directly.
  - World::new pre-generates chunks at cy in -1..=2 (4 vertical levels
    of pre-gen, ~1.1k chunks total, ~4.6 MB) covering the natural
    surface band. Above/below grows lazily.
  - generate_chunk takes the full 3D chunk coord; computes oy, fills
    blocks for this slice of the column. Trees that straddle a chunk
    boundary deterministically fill in their portion (each chunk uses
    the same hash, writes only its slice).
  - natural_surface_y returns the topmost-solid Y (preserved old
    semantic so all collision / spawn tests still hold).
  - 2 new tests pin the lazy-chunk-creation invariant (above and
    below pre-gen ranges).

mesh.rs:
  - dims is [CHUNK_SIZE; 3]; iteration loops are cubic.
  - base_y added so neighbor lookups + corner positions account for
    the chunk's vertical offset.
  - warm_heightmaps_around takes (cx, cz) column coords now.

sim/spawn.rs:
  - find_safe_spawn no longer caps at CHUNK_HEIGHT-2. Safety bound
    is surface_y + 200 (pathological tower depth before giving up).

sim/lighting.rs:
  - SUN_RAY_TOP_Y bumped from 128 to 4096 (was matching the old
    fixed world height). Ray DDA still terminates at the cap.

sim/visibility.rs:
  - Chunk AABB is CHUNK_SIZE on all three axes; the chunk's cy now
    contributes to the AABB min Y (was hardcoded 0).

render/mod.rs:
  - Heightmap warming + comment updates for column-keyed semantics.

Tests: 65/65 pass (was 63, +2 new). Native + wasm release clean.
Smoke-tested live: 88 fps, alive, spawn at y=8. All four UI
scenarios pass.

This is the foundation for the LOD pyramid you directed earlier —
isotropic chunks downsample cleanly (a LOD-N chunk represents a
(2^N)³ region with 16³ entries), and unbounded Y lets the LOD
tree extend in any direction without special cases.

src/mesh.rs

@@ -1,15 +1,17 @@
-use crate::world::{Block, Chunk, Face, World, CHUNK_HEIGHT, CHUNK_SIZE};
+use crate::world::{Block, Chunk, Face, World, CHUNK_SIZE};
 use bytemuck::{Pod, Zeroable};
 use glam::{IVec3, Vec3};
 
-/// Ensure heightmaps for this chunk and its 8 neighbors are cached,
+/// Ensure heightmaps for this chunk's (cx, cz) column + its 8 neighbor
-/// so the per-vertex ambience bake can do O(1) column lookups via
+/// columns are cached so the per-vertex ambience bake can do O(1)
-/// `world.column_top_y(..)`. Run once per chunk at the start of
+/// `column_top_y` lookups. Idempotent — recomputes only uncached
-/// `build_chunk_mesh`. Idempotent — only computes uncached entries.
+/// columns. With cubic chunks, the heightmap is keyed on the
+/// horizontal column, not on the chunk's vertical level — one
+/// heightmap serves every cubic chunk in the column.
 pub fn warm_heightmaps_around(world: &mut World, coord: IVec3) {
     for dx in -1..=1 {
         for dz in -1..=1 {
-            let _ = world.heightmap(IVec3::new(coord.x + dx, 0, coord.z + dz));
+            let _ = world.heightmap(coord.x + dx, coord.z + dz);
         }
     }
 }
@@ -108,9 +110,11 @@ pub fn build_chunk_mesh(world: &World, chunk: &Chunk) -> ChunkMesh {
     let mut vertices: Vec<Vertex> = Vec::with_capacity(2048);
     let mut indices: Vec<u32> = Vec::with_capacity(3072);
     let base_x = chunk.coord.x * CHUNK_SIZE;
+    let base_y = chunk.coord.y * CHUNK_SIZE;
     let base_z = chunk.coord.z * CHUNK_SIZE;
 
-    let dims = [CHUNK_SIZE, CHUNK_HEIGHT, CHUNK_SIZE];
+    // Cubic chunks — every axis is CHUNK_SIZE.
+    let dims = [CHUNK_SIZE, CHUNK_SIZE, CHUNK_SIZE];
 
     for face in Face::ALL {
         let normal = face.normal();
@@ -149,20 +153,28 @@ pub fn build_chunk_mesh(world: &World, chunk: &Chunk) -> ChunkMesh {
                     let nx = p[0] + normal.x;
                     let ny = p[1] + normal.y;
                     let nz = p[2] + normal.z;
-                    let neighbor_solid = if ny < 0 || ny >= CHUNK_HEIGHT {
+                    // Neighbor lookup: in-chunk if all locals in
-                        false
+                    // [0, CHUNK_SIZE); otherwise query the world,
-                    } else if nx >= 0 && nx < CHUNK_SIZE && nz >= 0 && nz < CHUNK_SIZE {
+                    // which may cross into a vertically adjacent
+                    // chunk (the new cubic case).
+                    let in_chunk = nx >= 0
+                        && nx < CHUNK_SIZE
+                        && ny >= 0
+                        && ny < CHUNK_SIZE
+                        && nz >= 0
+                        && nz < CHUNK_SIZE;
+                    let neighbor_solid = if in_chunk {
                         chunk.get(nx, ny, nz).solid()
                     } else {
                         world
-                            .get_block(IVec3::new(base_x + nx, ny, base_z + nz))
+                            .get_block(IVec3::new(base_x + nx, base_y + ny, base_z + nz))
                             .solid()
                     };
                     if neighbor_solid {
                         continue;
                     }
                     // World-space block position.
-                    let block_world = IVec3::new(base_x + p[0], p[1], base_z + p[2]);
+                    let block_world = IVec3::new(base_x + p[0], base_y + p[1], base_z + p[2]);
                     let face_plane = block_world + normal;
                     // 4 corner AO values, ordered (min-u min-v), (max-u min-v),
                     // (max-u max-v), (min-u max-v).
@@ -206,7 +218,13 @@ pub fn build_chunk_mesh(world: &World, chunk: &Chunk) -> ChunkMesh {
                             p[axis] = slice as f32;
                             p[u_axis] = u_val as f32;
                             p[v_axis] = v_val as f32;
-                            [p[0] + base_x as f32, p[1], p[2] + base_z as f32]
+                            // All three axes contribute a chunk-base
+                            // offset now that chunks are cubic.
+                            [
+                                p[0] + base_x as f32,
+                                p[1] + base_y as f32,
+                                p[2] + base_z as f32,
+                            ]
                         };
                         let c0 = to_world(u0, v0);
                         let c1 = to_world(u0 + w, v0);
@@ -415,7 +433,7 @@ pub fn name_hash(s: &str) -> u32 {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::world::{Block, Chunk, World, CHUNK_HEIGHT, CHUNK_SIZE};
+    use crate::world::{Block, Chunk, World, CHUNK_SIZE};
 
     /// A world containing exactly one chunk at the origin with all blocks
     /// you put into it via the closure, and nothing else.
@@ -570,8 +588,8 @@ mod tests {
     // is being rearranged.
     #[test]
     fn world_constants_are_sane() {
+        // Cubic chunks: just one CHUNK_SIZE for all three axes.
         assert!(CHUNK_SIZE > 0);
-        assert!(CHUNK_HEIGHT > 0);
     }
 
     // ---------- emit_oriented_box ----------

src/render/mod.rs

@@ -495,10 +495,11 @@ impl Renderer {
     }
 
     pub fn rebuild_chunk(&mut self, coord: IVec3, world: &mut World) {
-        // Heightmaps for this chunk + 8 neighbors are required by the
+        // Heightmaps for this chunk's column + 8 neighbor columns are
-        // fast ambience bake. Cheap (O(N²·CHUNK_HEIGHT) per chunk,
+        // required by the fast ambience bake. Cheap (one scan across
-        // cached). Called before build_chunk_mesh, never lazily inside
+        // the column's stacked chunks, cached). Called before
-        // it, so the build path stays free of `&mut World`.
+        // build_chunk_mesh, never lazily inside it, so the build path
+        // stays free of `&mut World`.
         warm_heightmaps_around(world, coord);
         let Some(chunk) = world.chunks.get(&coord) else {
             return;

src/sim/lighting.rs

@@ -16,8 +16,11 @@ pub const DAY_PERIOD: f32 = 300.0;
 /// Phase offset so a fresh world starts at noon (0.25 cycles past dawn).
 pub const SUN_OFFSET: f32 = 0.25;
 /// Y above which the sun-occlusion ray is considered to have escaped
-/// the world (open sky). Must be ≥ `world::CHUNK_HEIGHT`.
+/// the world (open sky). Since cubic chunks now extend Y indefinitely,
-pub const SUN_RAY_TOP_Y: i32 = 128;
+/// this is just a "we've gone too high to keep looking" cap — large
+/// enough that any reasonable structure is below it but not unbounded
+/// so degenerate rays still terminate.
+pub const SUN_RAY_TOP_Y: i32 = 4096;
 /// Hard cap on DDA steps so a degenerate near-horizontal ray can't loop
 /// forever. 512 voxels is well past any reasonable world width.
 const SUN_RAY_MAX_STEPS: u32 = 512;

src/sim/spawn.rs

@@ -1,7 +1,7 @@
 //! Spawn-point selection and fall-damage. Both are pure functions of
 //! world state and a scalar input, so they're easy to pin with
 //! regression tests.
-use crate::world::{natural_surface_y, World, CHUNK_HEIGHT};
+use crate::world::{natural_surface_y, World};
 use glam::{IVec3, Vec3};
 
 /// Returns the player feet position to spawn at. Anchored to the
@@ -13,7 +13,12 @@ pub fn find_safe_spawn(world: &World) -> Vec3 {
     let (x, z) = (0_i32, 0_i32);
     let surface_y = natural_surface_y(x, z);
     let mut feet_y = surface_y + 1;
-    let max_y = CHUNK_HEIGHT - 2;
+    // No CHUNK_HEIGHT bound anymore — cubic chunks stack indefinitely.
+    // We still cap the scan at +200 above the surface as a safety,
+    // since any tower that tall + a clear cap above would be
+    // pathological and any actual spawn would have found a slot well
+    // before there.
+    let max_y = surface_y + 200;
     while feet_y < max_y {
         let body_blocked = world.get_block(IVec3::new(x, feet_y, z)).solid()
             || world.get_block(IVec3::new(x, feet_y + 1, z)).solid();

src/sim/visibility.rs

@@ -1,7 +1,7 @@
 //! Frustum + radial culling for chunk visibility. Pure function on
 //! `(World, Camera, render_dist) -> Vec<ChunkCoord>`.
 use crate::camera::Camera;
-use crate::world::{World, CHUNK_HEIGHT, CHUNK_SIZE};
+use crate::world::{World, CHUNK_SIZE};
 use glam::{IVec3, Vec3};
 
 /// Returns the chunk coordinates whose AABB intersects the camera's
@@ -41,12 +41,15 @@ fn frustum_planes(camera: &Camera) -> [[f32; 4]; 6] {
 }
 
 fn chunk_in_frustum(coord: IVec3, planes: &[[f32; 4]; 6]) -> bool {
+    // Cubic chunks — all three axes are CHUNK_SIZE wide and the
+    // chunk's Y coordinate now matters (was always 0 in the old
+    // column-chunk model).
     let min = Vec3::new(
         (coord.x * CHUNK_SIZE) as f32,
-        0.0,
+        (coord.y * CHUNK_SIZE) as f32,
         (coord.z * CHUNK_SIZE) as f32,
     );
-    let max = min + Vec3::new(CHUNK_SIZE as f32, CHUNK_HEIGHT as f32, CHUNK_SIZE as f32);
+    let max = min + Vec3::splat(CHUNK_SIZE as f32);
     for p in planes {
         // p-vertex (the AABB corner furthest along plane normal).
         let px = if p[0] > 0.0 { max.x } else { min.x };

src/world.rs

@@ -1,10 +1,18 @@
-use glam::{IVec3, Vec3};
+use glam::{IVec2, IVec3, Vec3};
 use std::collections::HashMap;
 
 pub const CHUNK_SIZE: i32 = 16;
-pub const CHUNK_HEIGHT: i32 = 64;
 pub const WORLD_RADIUS: i32 = 8;
 
+/// Vertical range of chunks pre-generated around the surface, in chunk
+/// units (so 4 = 64 blocks of vertical pre-gen, centered on the terrain).
+/// The world has no hard Y limit — players can dig below or build above
+/// this range, and `set_block` lazily creates chunks on demand. The
+/// pre-gen range exists so the world has actual terrain visible on
+/// init without waiting for streaming.
+pub const PRE_GEN_BOTTOM_CHUNK_Y: i32 = -1;
+pub const PRE_GEN_TOP_CHUNK_Y: i32 = 2;
+
 #[repr(u8)]
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 pub enum Block {
@@ -115,6 +123,11 @@ impl Face {
     }
 }
 
+/// A cubic 16×16×16 voxel chunk. The chunk grid extends in all three
+/// axes — `coord.y` is the vertical chunk index, so the world has no
+/// fixed top or bottom. Chunks are created lazily on edit (see
+/// `World::set_block`) so building straight up doesn't need any
+/// pre-allocation.
 #[derive(Clone)]
 pub struct Chunk {
     pub blocks: Vec<Block>,
@@ -125,26 +138,29 @@ pub struct Chunk {
 impl Chunk {
     pub fn new(coord: IVec3) -> Self {
         Self {
-            blocks: vec![Block::Air; (CHUNK_SIZE * CHUNK_HEIGHT * CHUNK_SIZE) as usize],
+            blocks: vec![Block::Air; (CHUNK_SIZE * CHUNK_SIZE * CHUNK_SIZE) as usize],
             coord,
             dirty: true,
         }
     }
 
+    /// Flatten (x, y, z) within this chunk into the linear array index.
+    /// Layout is z-major then y then x; chosen so iterating x last is
+    /// cache-friendly for the greedy-mesh axis loops.
     #[inline]
     pub fn index(x: i32, y: i32, z: i32) -> usize {
         ((y * CHUNK_SIZE + z) * CHUNK_SIZE + x) as usize
     }
 
     pub fn get(&self, x: i32, y: i32, z: i32) -> Block {
-        if x < 0 || x >= CHUNK_SIZE || y < 0 || y >= CHUNK_HEIGHT || z < 0 || z >= CHUNK_SIZE {
+        if x < 0 || x >= CHUNK_SIZE || y < 0 || y >= CHUNK_SIZE || z < 0 || z >= CHUNK_SIZE {
             return Block::Air;
         }
         self.blocks[Self::index(x, y, z)]
     }
 
     pub fn set(&mut self, x: i32, y: i32, z: i32, b: Block) {
-        if x < 0 || x >= CHUNK_SIZE || y < 0 || y >= CHUNK_HEIGHT || z < 0 || z >= CHUNK_SIZE {
+        if x < 0 || x >= CHUNK_SIZE || y < 0 || y >= CHUNK_SIZE || z < 0 || z >= CHUNK_SIZE {
             return;
         }
         self.blocks[Self::index(x, y, z)] = b;
@@ -152,13 +168,14 @@ impl Chunk {
     }
 }
 
-/// Per-chunk topmost-solid-Y map: `heights[z * CHUNK_SIZE + x]` =
+/// Per-column top-of-solid map. Keyed by horizontal chunk coord
-/// highest world-Y at which `(chunk_x*16+x, y, chunk_z*16+z)` is a
+/// `(cx, cz)` and indexed by chunk-local `(lx, lz)` in `0..CHUNK_SIZE`.
-/// solid voxel. `i32::MIN` if no solid in the column.
+/// Each entry is the highest world-Y at which `(cx*16+lx, *, cz*16+lz)`
+/// contains a solid voxel, across all vertical chunks at that column.
+/// `i32::MIN` if the column has no solid block anywhere.
 ///
-/// Computed in O(N²·CHUNK_HEIGHT) once per chunk and cached so the
+/// Used by `sim::lighting::compute_ambience_fast` to do O(1) lookups
-/// sky_visibility bake can do O(1) array lookups instead of casting
+/// instead of casting hemisphere rays. Recomputed on edit per column.
-/// hemisphere rays. Recomputed on edit via `Chunk::dirty_heightmap`.
 #[derive(Clone, Debug)]
 pub struct HeightMap {
     pub heights: Vec<i32>,
@@ -171,8 +188,6 @@ impl HeightMap {
         }
     }
 
-    /// World-space `(x, z)` highest solid Y in this chunk. Caller must
-    /// convert to chunk-local coords (0..CHUNK_SIZE).
     #[inline]
     pub fn get_local(&self, lx: i32, lz: i32) -> i32 {
         if lx < 0 || lx >= CHUNK_SIZE || lz < 0 || lz >= CHUNK_SIZE {
@@ -181,19 +196,36 @@ impl HeightMap {
         self.heights[(lz * CHUNK_SIZE + lx) as usize]
     }
 
-    pub fn from_chunk(chunk: &Chunk) -> Self {
+    /// Build the heightmap by scanning every chunk at `(cx, *, cz)`
+    /// top-down for the topmost solid block. Cubic chunks mean we
+    /// can't assume a single Y range; we just look at whatever
+    /// chunks exist in that column.
+    pub fn from_world_column(world: &World, cx: i32, cz: i32) -> Self {
         let mut h = Self::new();
-        for z in 0..CHUNK_SIZE {
+        // Collect cy values that exist in this column, then process
-            for x in 0..CHUNK_SIZE {
+        // top-down so we early-exit per (lx, lz) on first solid hit.
+        let mut cy_list: Vec<i32> = world
+            .chunks
+            .keys()
+            .filter(|c| c.x == cx && c.z == cz)
+            .map(|c| c.y)
+            .collect();
+        cy_list.sort_unstable_by(|a, b| b.cmp(a)); // descending
+
+        for lx in 0..CHUNK_SIZE {
+            for lz in 0..CHUNK_SIZE {
                 let mut top = i32::MIN;
-                // Scan top-down so we early-exit on first solid.
+                'cy: for cy in &cy_list {
-                for y in (0..CHUNK_HEIGHT).rev() {
+                    let coord = IVec3::new(cx, *cy, cz);
-                    if chunk.blocks[Chunk::index(x, y, z)].solid() {
+                    let Some(chunk) = world.chunks.get(&coord) else { continue };
-                        top = y;
+                    for ly in (0..CHUNK_SIZE).rev() {
-                        break;
+                        if chunk.blocks[Chunk::index(lx, ly, lz)].solid() {
+                            top = cy * CHUNK_SIZE + ly;
+                            break 'cy;
                         }
                     }
-                h.heights[(z * CHUNK_SIZE + x) as usize] = top;
+                }
+                h.heights[(lz * CHUNK_SIZE + lx) as usize] = top;
             }
         }
         h
@@ -208,12 +240,8 @@ impl Default for HeightMap {
 
 pub struct World {
     pub chunks: HashMap<IVec3, Chunk>,
-    /// Lazily-built per-chunk heightmap cache. `World::heightmap()`
+    /// Per-(cx, cz) column heightmap cache. Invalidated on `set_block`.
-    /// computes-and-caches; mesh rebuilds invalidate via `set_block`.
+    pub heightmaps: HashMap<IVec2, HeightMap>,
-    /// Kept on `World` rather than `Chunk` so it can be recomputed in
-    /// a single immutable-borrow pass without aliasing the chunks
-    /// HashMap.
-    pub heightmaps: HashMap<IVec3, HeightMap>,
 }
 
 impl World {
@@ -221,62 +249,58 @@ impl World {
         let mut chunks = HashMap::new();
         for cx in -WORLD_RADIUS..=WORLD_RADIUS {
             for cz in -WORLD_RADIUS..=WORLD_RADIUS {
-                let coord = IVec3::new(cx, 0, cz);
+                for cy in PRE_GEN_BOTTOM_CHUNK_Y..=PRE_GEN_TOP_CHUNK_Y {
+                    let coord = IVec3::new(cx, cy, cz);
                     let chunk = generate_chunk(coord);
                     chunks.insert(coord, chunk);
                 }
             }
+        }
         Self {
             chunks,
             heightmaps: HashMap::new(),
         }
     }
 
-    /// Get (or compute + cache) the heightmap for `chunk_coord`. Used
+    /// Lookup or compute the heightmap for column `(cx, cz)`.
-    /// by the sky-visibility bake to do O(1) column lookups instead
+    pub fn heightmap(&mut self, cx: i32, cz: i32) -> &HeightMap {
-    /// of casting hemisphere rays.
+        let key = IVec2::new(cx, cz);
-    pub fn heightmap(&mut self, chunk_coord: IVec3) -> &HeightMap {
+        if !self.heightmaps.contains_key(&key) {
-        if !self.heightmaps.contains_key(&chunk_coord) {
+            let h = HeightMap::from_world_column(self, cx, cz);
-            if let Some(chunk) = self.chunks.get(&chunk_coord) {
+            self.heightmaps.insert(key, h);
-                let h = HeightMap::from_chunk(chunk);
-                self.heightmaps.insert(chunk_coord, h);
-            } else {
-                self.heightmaps.insert(chunk_coord, HeightMap::new());
         }
-        }
+        self.heightmaps.get(&key).unwrap()
-        self.heightmaps.get(&chunk_coord).unwrap()
-    }
-
-    /// Read-only heightmap fetch — returns a borrowed `Option<&HeightMap>`
-    /// without computing on miss. The bake path uses this after
-    /// `heightmap()` has populated the cache.
-    pub fn heightmap_get(&self, chunk_coord: IVec3) -> Option<&HeightMap> {
-        self.heightmaps.get(&chunk_coord)
     }
 
     /// World-coords helper: get the topmost solid Y at world column
-    /// `(wx, wz)`. Returns `i32::MIN` if no solid (open sky all the
+    /// `(wx, wz)`. Returns `i32::MIN` if the column has no solid block
-    /// way down) or if the column's chunk hasn't been heightmapped.
+    /// or hasn't been heightmapped yet (caller should invoke
+    /// `heightmap(cx, cz)` first to warm the cache).
     pub fn column_top_y(&self, wx: i32, wz: i32) -> i32 {
-        let (cc, lc) = Self::block_to_chunk(IVec3::new(wx, 0, wz));
+        let cx = wx.div_euclid(CHUNK_SIZE);
-        match self.heightmaps.get(&cc) {
+        let cz = wz.div_euclid(CHUNK_SIZE);
-            Some(h) => h.get_local(lc.x, lc.z),
+        let lx = wx.rem_euclid(CHUNK_SIZE);
+        let lz = wz.rem_euclid(CHUNK_SIZE);
+        match self.heightmaps.get(&IVec2::new(cx, cz)) {
+            Some(h) => h.get_local(lx, lz),
             None => i32::MIN,
         }
     }
 
+    /// Split a world block position into (chunk_coord, local_coord).
+    /// Both axes are 3D now — chunks stack vertically as well as
+    /// horizontally.
     pub fn block_to_chunk(pos: IVec3) -> (IVec3, IVec3) {
         let cx = pos.x.div_euclid(CHUNK_SIZE);
+        let cy = pos.y.div_euclid(CHUNK_SIZE);
         let cz = pos.z.div_euclid(CHUNK_SIZE);
         let lx = pos.x.rem_euclid(CHUNK_SIZE);
+        let ly = pos.y.rem_euclid(CHUNK_SIZE);
         let lz = pos.z.rem_euclid(CHUNK_SIZE);
-        (IVec3::new(cx, 0, cz), IVec3::new(lx, pos.y, lz))
+        (IVec3::new(cx, cy, cz), IVec3::new(lx, ly, lz))
     }
 
     pub fn get_block(&self, pos: IVec3) -> Block {
-        if pos.y < 0 || pos.y >= CHUNK_HEIGHT {
-            return Block::Air;
-        }
         let (c, l) = Self::block_to_chunk(pos);
         match self.chunks.get(&c) {
             Some(chunk) => chunk.get(l.x, l.y, l.z),
@@ -284,14 +308,15 @@ impl World {
         }
     }
 
+    /// Set a block at `pos`. Creates the containing chunk if it doesn't
+    /// exist (so building straight up creates chunks on demand —
+    /// there's no fixed Y ceiling). Returns whether the world changed.
     pub fn set_block(&mut self, pos: IVec3, b: Block) -> bool {
-        if pos.y < 0 || pos.y >= CHUNK_HEIGHT {
-            return false;
-        }
         let (c, l) = Self::block_to_chunk(pos);
-        let Some(chunk) = self.chunks.get_mut(&c) else {
+        let chunk = self
-            return false;
+            .chunks
-        };
+            .entry(c)
+            .or_insert_with(|| Chunk::new(c));
         chunk.set(l.x, l.y, l.z, b);
         // Mark neighbors dirty too so face culling is correct.
         for face in Face::ALL {
@@ -303,10 +328,8 @@ impl World {
                 }
             }
         }
-        // Heightmap is now stale for this chunk; drop the cached
+        // Invalidate the column heightmap.
-        // entry so the next bake recomputes. Neighbor heightmaps
+        self.heightmaps.remove(&IVec2::new(c.x, c.z));
-        // are unaffected because columns are chunk-local.
-        self.heightmaps.remove(&c);
         true
     }
 
@@ -402,15 +425,17 @@ fn value_noise(x: f32, z: f32) -> f32 {
     a + (b - a) * v
 }
 
-/// The y of the topmost natural-terrain block (i.e. ignoring any
+/// The y of the topmost natural-terrain block (ignoring player edits)
-/// player-placed edits) at world column `(x, z)`. The block *at* this y
+/// at world column `(x, z)`. Pure function of noise — no chunk lookup,
-/// is solid; `y + 1` is the first air block above the natural surface.
+/// no Y bound. The block AT this y is solid; `y + 1` is the first air
+/// block above the natural surface. Same semantics as the
+/// pre-cubic-chunks version so all collision/spawn tests still hold.
 pub fn natural_surface_y(x: i32, z: i32) -> i32 {
     let wx = x as f32;
     let wz = z as f32;
     let n = fbm(wx * 0.04, wz * 0.04);
     let height = (20.0 + n * 12.0).round() as i32;
-    height.clamp(1, CHUNK_HEIGHT - 1) - 1
+    height - 1
 }
 
 fn fbm(x: f32, z: f32) -> f32 {
@@ -427,46 +452,70 @@ fn fbm(x: f32, z: f32) -> f32 {
     sum / norm
 }
 
+/// Procedurally generate one cubic chunk. Determined entirely by the
+/// chunk's world coord — the same coord always produces the same chunk.
+/// Trees that straddle chunk boundaries deterministically fill in their
+/// portion of this chunk; the neighboring chunk fills the rest.
 fn generate_chunk(coord: IVec3) -> Chunk {
     let mut chunk = Chunk::new(coord);
     let ox = coord.x * CHUNK_SIZE;
+    let oy = coord.y * CHUNK_SIZE;
     let oz = coord.z * CHUNK_SIZE;
+
     for x in 0..CHUNK_SIZE {
         for z in 0..CHUNK_SIZE {
-            let wx = (ox + x) as f32;
+            // surface_y is the topmost SOLID block — same semantic
-            let wz = (oz + z) as f32;
+            // as `natural_surface_y`. Block at wy = surface_y is the
-            let n = fbm(wx * 0.04, wz * 0.04);
+            // grass/sand cap; wy > surface_y is air.
-            let height = (20.0 + n * 12.0).round() as i32;
+            let surface_y = natural_surface_y(ox + x, oz + z);
-            let height = height.clamp(1, CHUNK_HEIGHT - 1);
+
-            for y in 0..height {
+            // Terrain fill within this cubic chunk's Y range.
-                let b = if y == height - 1 {
+            for ly in 0..CHUNK_SIZE {
-                    if height < 18 {
+                let wy = oy + ly;
-                        Block::Sand
+                if wy > surface_y {
-                    } else {
+                    break;
-                        Block::Grass
                 }
-                } else if y > height - 4 {
+                let b = if wy == surface_y {
+                    if surface_y < 17 { Block::Sand } else { Block::Grass }
+                } else if wy > surface_y - 3 {
                     Block::Dirt
                 } else {
                     Block::Stone
                 };
-                chunk.set(x, y, z, b);
+                chunk.set(x, ly, z, b);
             }
-            // Occasional tree
+
+            // Trees, deterministic on (world_x, world_z). The trunk
+            // and leaves may span chunk boundaries; each chunk writes
+            // only the slice that falls within its own Y range. The
+            // neighboring chunk fills the rest using the same hash.
             let tree_hash = hash2(ox + x + 1000, oz + z - 1000);
-            if tree_hash > 0.93 && height >= 18 && height < CHUNK_HEIGHT - 8 {
+            if tree_hash > 0.93 && surface_y >= 17 {
+                // Trunk: 4 wood blocks at (surface_y+1) .. (surface_y+5)
+                let trunk_base = surface_y + 1;
                 for ty in 0..4 {
-                    chunk.set(x, height + ty, z, Block::Wood);
+                    let wy = trunk_base + ty;
+                    if wy >= oy && wy < oy + CHUNK_SIZE {
+                        chunk.set(x, wy - oy, z, Block::Wood);
                     }
+                }
+                // Leaves: ball around y = trunk_base + 3
                 for dx in -2..=2_i32 {
                     for dz in -2..=2_i32 {
-                        for dy in 3..=5_i32 {
+                        for dy in 2..=4_i32 {
-                            if dx.abs() + dz.abs() + (dy - 4).abs() <= 3 {
+                            if dx.abs() + dz.abs() + (dy - 3).abs() <= 3 {
                                 let lx = x + dx;
                                 let lz = z + dz;
-                                if lx >= 0 && lx < CHUNK_SIZE && lz >= 0 && lz < CHUNK_SIZE {
+                                let wy = trunk_base + dy;
-                                    let ly = height + dy;
+                                if lx >= 0
-                                    if ly < CHUNK_HEIGHT && chunk.get(lx, ly, lz) == Block::Air {
+                                    && lx < CHUNK_SIZE
+                                    && lz >= 0
+                                    && lz < CHUNK_SIZE
+                                    && wy >= oy
+                                    && wy < oy + CHUNK_SIZE
+                                {
+                                    let ly = wy - oy;
+                                    if chunk.get(lx, ly, lz) == Block::Air {
                                         chunk.set(lx, ly, lz, Block::Leaves);
                                     }
                                 }
@@ -494,11 +543,14 @@ mod tests {
     }
 
     #[test]
-    fn natural_surface_y_in_range() {
+    fn natural_surface_y_in_reasonable_range() {
+        // No more CHUNK_HEIGHT clamp — the surface is whatever the
+        // noise says. For our amplitude (20 ± 12) it should stay in
+        // a small band; we just sanity-check it's not absurd.
         for x in -200..=200 {
             for z in -200..=200 {
                 let y = natural_surface_y(x, z);
-                assert!((0..CHUNK_HEIGHT).contains(&y), "out of range at ({},{})", x, z);
+                assert!((0..64).contains(&y), "surface y={} out of expected band at ({},{})", y, x, z);
             }
         }
     }
@@ -510,7 +562,7 @@ mod tests {
         let origin = Vec3::new(0.5, surface as f32 + 12.0, 0.5);
         let hit = world.raycast(origin, Vec3::new(0.0, -1.0, 0.0), 30.0);
         let (hit_pos, _) = hit.expect("ray fired down at terrain must hit");
-        assert_eq!(hit_pos.y, surface, "ray must hit topmost solid block");
+        assert_eq!(hit_pos.y, surface, "ray must hit the topmost solid block (== surface)");
     }
 
     #[test]
@@ -533,4 +585,26 @@ mod tests {
         let manhattan = delta.x.abs() + delta.y.abs() + delta.z.abs();
         assert_eq!(manhattan, 1, "prev must be one block-step from hit");
     }
+
+    #[test]
+    fn set_block_creates_chunks_above_pregenerated() {
+        // The pre-gen range stops at CHUNK_SIZE * (PRE_GEN_TOP_CHUNK_Y + 1) - 1.
+        // Placing a block well above that should succeed and create the chunk.
+        let mut world = World::new();
+        let high_y = (PRE_GEN_TOP_CHUNK_Y + 5) * CHUNK_SIZE + 3;
+        let pos = IVec3::new(0, high_y, 0);
+        let ok = world.set_block(pos, Block::Stone);
+        assert!(ok);
+        assert_eq!(world.get_block(pos), Block::Stone);
+    }
+
+    #[test]
+    fn set_block_creates_chunks_below_pregenerated() {
+        let mut world = World::new();
+        let deep_y = (PRE_GEN_BOTTOM_CHUNK_Y - 5) * CHUNK_SIZE - 3;
+        let pos = IVec3::new(0, deep_y, 0);
+        let ok = world.set_block(pos, Block::Stone);
+        assert!(ok);
+        assert_eq!(world.get_block(pos), Block::Stone);
+    }
 }