Marching cubes
An algorithm that turns a 3D scalar field into a polygon mesh by deciding, cube by cube, where a surface crosses the grid.
Marching cubes, published by Lorensen and Cline at GE in 1987, extracts a surface (an "isosurface") from volumetric data. It walks a 3D grid one cube at a time; each of the eight corners is inside or outside the surface, giving 256 cases (reduced by symmetry to 15) that map, via a lookup table, to a set of triangles threaded through that cube. Stitched together, the cubes form a continuous mesh.
Born for medical and scientific visualization (CT and MRI slices), it became the standard way games build meshes from voxel or signed distance data — destructible terrain, smooth voxel worlds, metaballs. Unlike a heightmap it handles caves and overhangs because it works on a full volume, not one height per column. LOD schemes vary the cube size by distance.