Boulder

Boulders are unique placeable 2×2 foreground objects, and are stationary when placed. When unsupported, they fall and begin to roll away. They pick up speed and damage 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. On contact with entities, they cause damage in the range of 60-80 per hit. Players take double / the damage. Boulders pierce through an unlimited number of entities.

Boulders always roll left, unless there is no space for them to go left, in which case they roll right.

Boulders roll either left or right based on their horizontal placement (placing boulders an odd width apart will make them roll in opposite directions). If there is no space to start rolling in the desired direction, the boulders will roll in the other direction.

Boulders that are hit with a pickaxe roll away from the side they were hit.

Boulder traps occur naturally in the Underground and Cavern layers, and consist of a single Boulder in the ceiling, supported by six (2×3) 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.

Tips

 * In, Boulder traps should be avoided at all costs, considering the deadly force of 280 damage on average.
 * Boulders will deal more damage to players than NPCs and enemies in Expert and Master Mode worlds.
 * Boulders are effective when used along with Grenades against The Destroyer and Eater of Worlds.
 * Using Boulders is an efficient way to kill NPCs, should one wish to do that (for example, for getting their unique drops).
 * Players can detect Boulder traps "in the wild" by wielding a mechanism item such as a Pressure Plate, by carrying a or its upgrade  or by using a Dangersense Potion. All except the last will allow the player to view placed wires, and the vertical wire with its 2×3 rectangle atop it is quite distinctive. Once detected, there are some basic ways to disarm them:
 * If approaching from above, the player can mine their way toward the top of the trap, and use a pickaxe to break the boulder from outside the sealed chamber.
 * From below, the player can make sure there is a slope downwards to the left, then trigger the plate from the right side and immediately dodge to the right.
 * Once the player has The Grand Design (or even just a Wire Cutter) they can easily disable the trap at a distance by cutting the vertical wire.
 * At full tilt, boulders roll faster than a running player with several speed boosts applied. Using a Hoverboard or Asphalt can help to overcome them.
 * Breaking the pressure plate will prevent the trap from triggering, but the boulder will remain above, and may be encountered later.
 * As with other traps, the mechanisms (but not the boulder) can be harvested for later use. The pressure plate and active stone can be taken with a pickaxe, but collecting the wire needs a Wire Cutter or upgrade thereof.
 * In Hardmode, the Active Stone Blocks do not change into their respective biome. Therefore, they can be much easier to spot and deactivate.

Trivia

 * Boulders are one of four non-accessories that are crafted at a Tinkerer's Workshop, the others being the Cell Phone, Multicolor Wrench, and The Grand Design.
 * Boulders are the only /  one of the two (the other being Rolling Cactus) 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 (or destroys them, if they cannot roll).
 * Boulders are able to deal damage to Target Dummies.

History
Rocher