diff --git a/src/document/terrains.ts b/src/document/terrains.ts
new file mode 100644
index 00000000..2b2a2350
--- /dev/null
+++ b/src/document/terrains.ts
@@ -0,0 +1,291 @@
+import { createOpaqueId } from "../core/ids";
+import type { Vec3 } from "../core/vector";
+
+export interface Terrain {
+  id: string;
+  kind: "terrain";
+  name?: string;
+  visible: boolean;
+  enabled: boolean;
+  position: Vec3;
+  sampleCountX: number;
+  sampleCountZ: number;
+  cellSize: number;
+  heights: number[];
+}
+
+export const DEFAULT_TERRAIN_VISIBLE = true;
+export const DEFAULT_TERRAIN_ENABLED = true;
+export const MIN_TERRAIN_SAMPLE_COUNT = 2;
+export const DEFAULT_TERRAIN_SAMPLE_COUNT_X = 9;
+export const DEFAULT_TERRAIN_SAMPLE_COUNT_Z = 9;
+export const DEFAULT_TERRAIN_CELL_SIZE = 1;
+export const DEFAULT_TERRAIN_HEIGHT = 0;
+
+function cloneVec3(vector: Vec3): Vec3 {
+  return {
+    x: vector.x,
+    y: vector.y,
+    z: vector.z
+  };
+}
+
+function areVec3Equal(left: Vec3, right: Vec3): boolean {
+  return left.x === right.x && left.y === right.y && left.z === right.z;
+}
+
+function assertFiniteVec3(vector: Vec3, label: string) {
+  if (!Number.isFinite(vector.x) || !Number.isFinite(vector.y) || !Number.isFinite(vector.z)) {
+    throw new Error(`${label} must be finite on every axis.`);
+  }
+}
+
+export function normalizeTerrainName(
+  name: string | null | undefined
+): string | undefined {
+  if (name === undefined || name === null) {
+    return undefined;
+  }
+
+  const trimmedName = name.trim();
+  return trimmedName.length === 0 ? undefined : trimmedName;
+}
+
+export function normalizeTerrainSampleCount(
+  value: number,
+  label: string
+): number {
+  if (!Number.isInteger(value) || value < MIN_TERRAIN_SAMPLE_COUNT) {
+    throw new Error(
+      `${label} must be an integer greater than or equal to ${MIN_TERRAIN_SAMPLE_COUNT}.`
+    );
+  }
+
+  return value;
+}
+
+export function normalizeTerrainCellSize(value: number): number {
+  if (!Number.isFinite(value) || value <= 0) {
+    throw new Error("Terrain cell size must be a positive finite number.");
+  }
+
+  return value;
+}
+
+export function createFlatTerrainHeights(
+  sampleCountX: number,
+  sampleCountZ: number,
+  height = DEFAULT_TERRAIN_HEIGHT
+): number[] {
+  const normalizedSampleCountX = normalizeTerrainSampleCount(
+    sampleCountX,
+    "Terrain sampleCountX"
+  );
+  const normalizedSampleCountZ = normalizeTerrainSampleCount(
+    sampleCountZ,
+    "Terrain sampleCountZ"
+  );
+
+  if (!Number.isFinite(height)) {
+    throw new Error("Terrain height samples must be finite.");
+  }
+
+  return new Array(normalizedSampleCountX * normalizedSampleCountZ).fill(height);
+}
+
+export function getTerrainSampleIndex(
+  terrain: Pick<Terrain, "sampleCountX" | "sampleCountZ">,
+  sampleX: number,
+  sampleZ: number
+): number {
+  if (
+    !Number.isInteger(sampleX) ||
+    sampleX < 0 ||
+    sampleX >= terrain.sampleCountX
+  ) {
+    throw new Error(`Terrain sampleX ${sampleX} is out of range.`);
+  }
+
+  if (
+    !Number.isInteger(sampleZ) ||
+    sampleZ < 0 ||
+    sampleZ >= terrain.sampleCountZ
+  ) {
+    throw new Error(`Terrain sampleZ ${sampleZ} is out of range.`);
+  }
+
+  return sampleZ * terrain.sampleCountX + sampleX;
+}
+
+export function getTerrainHeightAtSample(
+  terrain: Terrain,
+  sampleX: number,
+  sampleZ: number
+): number {
+  return terrain.heights[getTerrainSampleIndex(terrain, sampleX, sampleZ)] ?? 0;
+}
+
+export function getTerrainWorldSamplePosition(
+  terrain: Terrain,
+  sampleX: number,
+  sampleZ: number
+): Vec3 {
+  return {
+    x: terrain.position.x + sampleX * terrain.cellSize,
+    y: terrain.position.y + getTerrainHeightAtSample(terrain, sampleX, sampleZ),
+    z: terrain.position.z + sampleZ * terrain.cellSize
+  };
+}
+
+export function getTerrainBounds(terrain: Terrain): { min: Vec3; max: Vec3 } {
+  const width = (terrain.sampleCountX - 1) * terrain.cellSize;
+  const depth = (terrain.sampleCountZ - 1) * terrain.cellSize;
+  let minHeight = Number.POSITIVE_INFINITY;
+  let maxHeight = Number.NEGATIVE_INFINITY;
+
+  for (const height of terrain.heights) {
+    minHeight = Math.min(minHeight, height);
+    maxHeight = Math.max(maxHeight, height);
+  }
+
+  if (!Number.isFinite(minHeight) || !Number.isFinite(maxHeight)) {
+    minHeight = 0;
+    maxHeight = 0;
+  }
+
+  return {
+    min: {
+      x: terrain.position.x,
+      y: terrain.position.y + minHeight,
+      z: terrain.position.z
+    },
+    max: {
+      x: terrain.position.x + width,
+      y: terrain.position.y + maxHeight,
+      z: terrain.position.z + depth
+    }
+  };
+}
+
+function createDefaultTerrainPosition(
+  sampleCountX: number,
+  sampleCountZ: number,
+  cellSize: number
+): Vec3 {
+  return {
+    x: -((sampleCountX - 1) * cellSize) * 0.5,
+    y: 0,
+    z: -((sampleCountZ - 1) * cellSize) * 0.5
+  };
+}
+
+export function createTerrain(
+  overrides: Partial<
+    Pick<
+      Terrain,
+      | "id"
+      | "name"
+      | "visible"
+      | "enabled"
+      | "position"
+      | "sampleCountX"
+      | "sampleCountZ"
+      | "cellSize"
+      | "heights"
+    >
+  > = {}
+): Terrain {
+  const sampleCountX = normalizeTerrainSampleCount(
+    overrides.sampleCountX ?? DEFAULT_TERRAIN_SAMPLE_COUNT_X,
+    "Terrain sampleCountX"
+  );
+  const sampleCountZ = normalizeTerrainSampleCount(
+    overrides.sampleCountZ ?? DEFAULT_TERRAIN_SAMPLE_COUNT_Z,
+    "Terrain sampleCountZ"
+  );
+  const cellSize = normalizeTerrainCellSize(
+    overrides.cellSize ?? DEFAULT_TERRAIN_CELL_SIZE
+  );
+  const position = cloneVec3(
+    overrides.position ??
+      createDefaultTerrainPosition(sampleCountX, sampleCountZ, cellSize)
+  );
+  const heights =
+    overrides.heights !== undefined
+      ? [...overrides.heights]
+      : createFlatTerrainHeights(sampleCountX, sampleCountZ);
+  const visible = overrides.visible ?? DEFAULT_TERRAIN_VISIBLE;
+  const enabled = overrides.enabled ?? DEFAULT_TERRAIN_ENABLED;
+
+  assertFiniteVec3(position, "Terrain position");
+
+  if (typeof visible !== "boolean") {
+    throw new Error("Terrain visible must be a boolean.");
+  }
+
+  if (typeof enabled !== "boolean") {
+    throw new Error("Terrain enabled must be a boolean.");
+  }
+
+  if (heights.length !== sampleCountX * sampleCountZ) {
+    throw new Error(
+      `Terrain heights must contain exactly ${sampleCountX * sampleCountZ} samples.`
+    );
+  }
+
+  if (heights.some((height) => !Number.isFinite(height))) {
+    throw new Error("Terrain heights must remain finite.");
+  }
+
+  return {
+    id: overrides.id ?? createOpaqueId("terrain"),
+    kind: "terrain",
+    name: normalizeTerrainName(overrides.name),
+    visible,
+    enabled,
+    position,
+    sampleCountX,
+    sampleCountZ,
+    cellSize,
+    heights
+  };
+}
+
+export function cloneTerrain(terrain: Terrain): Terrain {
+  return createTerrain(terrain);
+}
+
+export function areTerrainsEqual(left: Terrain, right: Terrain): boolean {
+  return (
+    left.id === right.id &&
+    left.kind === right.kind &&
+    left.name === right.name &&
+    left.visible === right.visible &&
+    left.enabled === right.enabled &&
+    areVec3Equal(left.position, right.position) &&
+    left.sampleCountX === right.sampleCountX &&
+    left.sampleCountZ === right.sampleCountZ &&
+    left.cellSize === right.cellSize &&
+    left.heights.length === right.heights.length &&
+    left.heights.every((height, index) => height === right.heights[index])
+  );
+}
+
+export function compareTerrains(left: Terrain, right: Terrain): number {
+  const leftName = left.name ?? "";
+  const rightName = right.name ?? "";
+
+  if (leftName !== rightName) {
+    return leftName.localeCompare(rightName);
+  }
+
+  return left.id.localeCompare(right.id);
+}
+
+export function getTerrains(terrains: Record<string, Terrain>): Terrain[] {
+  return Object.values(terrains).sort(compareTerrains);
+}
+
+export function getTerrainKindLabel(): string {
+  return "Terrain";
+}