Boulder

Boulders are unique placeable 2x2 foreground objects, and are stationary when placed. After placement, when mined with a pickaxe or drill, or when the blocks supporting them are mined, they fall and roll away, picking up speed until they collide with blocks (at which point they break and disappear). Boulders can not be picked up again once placed.

Boulders will pierce through unlimited enemies, players, and NPCs until colliding with solid blocks, causing extreme damage, in the range of 60-150, to each (regardless of Armor and other defensive equipment). Boulders always roll left, unless there is no space for them to go left.

Boulder traps occur naturally in the Cavern layer, and consist of a single Boulder in the ceiling, supported by six (2x3) Active Stone Blocks Wired to Pressure Plates. If an enemy or player walks onto the Pressure Plate, the Boulder falls and damages any players or enemies it hits.

Trivia

 * Boulders are the only placeable objects larger than one single tile in size that exist in the foreground (with the possible exception of the Door, which is in the foreground when closed). All others are either single foreground blocks, or background Furniture items.
 * Boulders are the only placeable objects that cannot be picked up again once placed, as Hammers have no effect on them, and mining them with a Pickaxe or Drill only causes them to roll.
 * At full tilt, boulders roll faster than a player with Well Fed buff, Werewolf buff, full Shadow Armor, a Quick Aglet, a Quick Anklet of the Wind, and 3 other Quick reforged accessories, or Quick Hermes/Spectre Boots. The boulder cannot be avoided by running in almost any circumstances, nor can a player ever catch up to a rolling one via running.