Boulder

The Boulder is a placeable 2x2 block added in version 1.1. Whenever they are unsupported or you use a pickaxe on it, it will roll and deal massive damage.

As of version 1.1, boulder traps are found in caves. 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 a player or mob 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. As shown in the video, a boulder will always roll to the left if possible. When there's no space for it to go left, the boulder will roll right instead.