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. These square monsters kill you if you come in contact with them. You can drop a rock on one to cause an explosion. They blow up if they hit an amoeba.