diff --git a/src/mudlib/server.py b/src/mudlib/server.py
index a31caeb..8b0581f 100644
--- a/src/mudlib/server.py
+++ b/src/mudlib/server.py
@@ -2,6 +2,7 @@
 
 import asyncio
 import logging
+import pathlib
 import time
 from typing import cast
 
@@ -136,12 +137,16 @@ async def run_server() -> None:
     """Start the MUD telnet server."""
     global _world
 
-    # Generate world once at startup
-    log.info("generating world (seed=42, 1000x1000)...")
+    # Generate world once at startup (cached to build/ after first run)
+    cache_dir = pathlib.Path(__file__).resolve().parents[2] / "build"
+    log.info("loading world (seed=42, 1000x1000)...")
     t0 = time.monotonic()
-    _world = World(seed=42, width=1000, height=1000)
+    _world = World(seed=42, width=1000, height=1000, cache_dir=cache_dir)
     elapsed = time.monotonic() - t0
-    log.info("world generated in %.2fs", elapsed)
+    if _world.cached:
+        log.info("world loaded from cache in %.2fs", elapsed)
+    else:
+        log.info("world generated in %.2fs (cached for next run)", elapsed)
 
     # Inject world into command modules
     mudlib.commands.fly.world = _world
diff --git a/src/mudlib/world/terrain.py b/src/mudlib/world/terrain.py
index 58e4af7..47cee1c 100644
--- a/src/mudlib/world/terrain.py
+++ b/src/mudlib/world/terrain.py
@@ -1,6 +1,7 @@
 """Terrain generation for MUD worlds using deterministic noise."""
 
 import math
+import pathlib
 import random
 
 
@@ -70,14 +71,33 @@ class World:
         "water": "~",
     }
 
-    def __init__(self, seed: int = 42, width: int = 1000, height: int = 1000):
+    def __init__(
+        self,
+        seed: int = 42,
+        width: int = 1000,
+        height: int = 1000,
+        cache_dir: pathlib.Path | None = None,
+    ):
         self.seed = seed
         self.width = width
         self.height = height
+        self.cached = False
+
+        # try loading from cache
+        if cache_dir is not None:
+            terrain = self._load_cache(cache_dir)
+            if terrain is not None:
+                self.terrain = terrain
+                self.cached = True
+                return
 
         # generate terrain grid
         self.terrain = self._generate_terrain()
 
+        # save to cache for next time
+        if cache_dir is not None:
+            self._save_cache(cache_dir)
+
     def _generate_elevation(self) -> list[list[float]]:
         """Generate tileable elevation map using fractal noise.
 
@@ -249,6 +269,32 @@ class World:
         self._generate_rivers(terrain, elevation)
         return terrain
 
+    def _cache_path(self, cache_dir: pathlib.Path) -> pathlib.Path:
+        return cache_dir / f"terrain_{self.seed}_{self.width}x{self.height}.bin"
+
+    def _load_cache(self, cache_dir: pathlib.Path) -> list[list[str]] | None:
+        """Load terrain from a cached binary file. Returns None on miss."""
+        path = self._cache_path(cache_dir)
+        if not path.exists():
+            return None
+        data = path.read_bytes()
+        if len(data) != self.width * self.height:
+            return None
+        terrain = []
+        for y in range(self.height):
+            offset = y * self.width
+            row = [chr(b) for b in data[offset : offset + self.width]]
+            terrain.append(row)
+        return terrain
+
+    def _save_cache(self, cache_dir: pathlib.Path) -> None:
+        """Save terrain to a binary file (1 byte per tile)."""
+        cache_dir.mkdir(parents=True, exist_ok=True)
+        data = bytearray()
+        for row in self.terrain:
+            data.extend(ord(c) for c in row)
+        self._cache_path(cache_dir).write_bytes(bytes(data))
+
     def wrap(self, x: int, y: int) -> tuple[int, int]:
         """Wrap coordinates to stay within world bounds."""
         return x % self.width, y % self.height
diff --git a/tests/test_terrain.py b/tests/test_terrain.py
index 07f113d..9ba2964 100644
--- a/tests/test_terrain.py
+++ b/tests/test_terrain.py
@@ -204,3 +204,40 @@ def test_large_world_generation():
     assert world.get_tile(0, 0) in {".", "^", "~", "T", ":"}
     assert world.get_tile(500, 500) in {".", "^", "~", "T", ":"}
     assert world.get_tile(999, 999) in {".", "^", "~", "T", ":"}
+
+
+# --- terrain caching ---
+
+
+def test_world_saves_cache(tmp_path):
+    """World saves terrain cache when cache_dir is provided."""
+    world = World(seed=42, width=100, height=100, cache_dir=tmp_path)
+    cache_file = tmp_path / "terrain_42_100x100.bin"
+    assert cache_file.exists()
+    assert cache_file.stat().st_size == 100 * 100  # 1 byte per tile
+    assert not world.cached  # first run generates, doesn't load from cache
+
+
+def test_world_loads_from_cache(tmp_path):
+    """World loads terrain from cache on second creation."""
+    world1 = World(seed=42, width=100, height=100, cache_dir=tmp_path)
+    world2 = World(seed=42, width=100, height=100, cache_dir=tmp_path)
+
+    for y in range(100):
+        for x in range(100):
+            assert world1.get_tile(x, y) == world2.get_tile(x, y)
+
+    assert world2.cached
+
+
+def test_world_cache_different_params(tmp_path):
+    """Different seed/size doesn't use wrong cache."""
+    World(seed=42, width=100, height=100, cache_dir=tmp_path)
+    world2 = World(seed=99, width=100, height=100, cache_dir=tmp_path)
+    assert not world2.cached  # different seed, must generate
+
+
+def test_world_no_cache_by_default():
+    """World doesn't cache when no cache_dir provided."""
+    world = World(seed=42, width=50, height=50)
+    assert not world.cached