Boulder

Boulders are unique placeable 2x2 foreground objects, and are stationary when placed. When unsupported, they fall and begin to roll away (to the left, unless there are obstructions immediately to that side), picking up speed and damaging players, enemies and NPCs along their path. Boulders will also begin to roll if mined with a Pickaxe or Drill, meaning that they cannot be picked up again once placed. Boulders will roll across flat surfaces and down slopes, but will break and disappear on contact with an obstacle.

Boulders will pierce through unlimited entities until colliding with solid blocks, causing extreme damage in the range of 60-150 per hit regardless of Armor and other defensive equipment. Boulders always roll left, unless there is no space for them to go left, in which case they roll right.

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

Trivia

 * 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 Boots/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, unless the player uses a Hoverboard, or moves over Asphalt.
 * The boulder is the only item that requires a Tinkerer's Workshop to craft that isn't an Accessory.