It appears when you have gathered enough diamonds. Rocks fall when there's no ground underneath. If a falling rock lands on you, it kills you. You can push rocks, and rocks can be destroyed in an explosion. You can dig through the ground by walking through it.
Solid walls are indestructible.
Walls can be destroyed by an explosion. Some walls are expanding walls, which grow horizontally. Magic walls turn rocks to diamonds and diamonds to rocks, for a limited time. They don't reveal themselves until the first rock or diamond falls through. If there's no space beneath them, they destroy anything falling through.