A heightmap can only ever describe one elevation per point (bright is high, dark is low), which makes it incapable of a tunnel with rock above it. Real Roblox terrain is a true 3D voxel volume underneath, though, and there's a real API for carving it. This generates a network of tunnels, lets you place entrances, previews them in 3D against your terrain, and exports a script that carves them for real with Terrain:FillBall().