Boulder

The Boulder is a placeable 2x2 block added in 1.1 patch. Whenever they are unsupported or you use a pickaxe on it, it will roll and deal massive damage. If a rolling boulder hits a wall, it will break and give no drop. (bug?)

Boulder traps are naturally occurring in caves of worlds generated after 1.1 patch. These consist of a single boulder in the ceiling supported by several Active Stone Blocks and wired to a Pressure Plate. If the player mines the Active Stone Blocks or someone stands on the Pressure Plate, the boulder will fall and roll towards the player at a fast speed; if the player is hit, they will take massive damage, 80-150, which probably depends on speed.