Boulder

Boulder traps generate through the underground and cavern layers and they all look the same once you know the shape. A single boulder sits in the ceiling, held up by a small block of active stone, with a wire running down to an orange pressure plate on the floor below. Step on the plate, the stone deactivates, and the boulder drops and starts rolling.

That vertical wire with a small rectangle of stone perched on top is the giveaway. Holding any wiring item in your hand shows placed wire, as does a Mechanical Lens or The Grand Design. A Dangersense Potion flags the trap without showing you the wiring at all, which is the low effort option for a long mining trip.

Cutting the vertical wire kills the trap permanently and is the easy answer once you own a Wire Cutter or better, since you can do it from a distance. Breaking the pressure plate stops it triggering but leaves the boulder sitting up there, loaded, waiting for whoever digs into that ceiling later. Both mechanisms can be harvested and reused, though the wire itself needs cutters to collect.

Without wiring tools there are two manual approaches. From above, tunnel toward the top of the chamber and break the boulder with a pickaxe while standing outside it. From below, make sure the floor slopes away to one side, trigger the plate from the far side, and immediately move the same way the boulder is not going.

Speed does not change what it hits you for. Getting clipped by one that has just started moving hurts exactly as much as being run down at full tilt. The same rule means mining a boulder while you are standing against it will hurt you, though a boulder with nowhere to roll no longer punishes you for that.

Do not try to outrun it on foot. At full speed it moves faster than a player with several movement boosts stacked, and it only stops when it hits something solid. A Hoverboard or a strip of Asphalt Block can win that race. Otherwise put an obstacle in its path or get off the floor entirely, since it cannot leave the surface it is rolling on. It will climb a slope if the terrain gives it a flat block between each sloped one, so uphill is not automatically safe.

Expert and Master turn this from a nasty surprise into something that can simply end a run, and defense only started reducing the damage at all a few versions in. Treat every unexplained pressure plate underground as one of these until proven otherwise.

Once a boulder is down you cannot take it back. Hammers do nothing to it, and hitting it with a pickaxe or drill only sets it rolling, or destroys it outright if it has nowhere to go. It is one of a very short list of placeable objects with that property, alongside the rolling cactus, the bee hive and the bouncy and life crystal variants.

They can be stacked on each other and set on top of flat furniture like tables and bookcases. Mining the bottom one of a stack sends the whole column down at once, while mining the top one only releases that single boulder, which is the basis for most player built boulder traps.

The direction is not random, which matters if you are building with them. A boulder started with a pickaxe rolls toward whichever side gives it the longer run, so placing it nearer one wall sends it away from that wall. A boulder produced by a Boulder Statue picks its direction from whether its horizontal coordinate is odd or even, so if a statue is throwing them the wrong way, move the statue one tile across and it flips.

What you get for the one way commitment is unlimited piercing, very heavy knockback, and a projectile that keeps going for a full minute before it expires. Rolled down a long flat corridor that is over a thousand tiles of travel.

The classic use is worm bosses. A boulder run combined with Grenades strips segments off The Destroyer and the Eater of Worlds faster than most early weapons manage. They are also the standard method when somebody wants a town NPC dead for a unique drop, since the boulder does the work without you attacking anyone directly. Boulders will trigger the NPC sensing pressure plates as they pass, and they even register on target dummies.

In For the Worthy and Zenith worlds a boulder does not simply disappear when it breaks. It splits into a few smaller ones that behave identically at reduced damage, which turns a single trap into a spray and makes the underground substantially nastier.

World generation also quietly marks a handful of boulders, two on a small world and up to six on a large one, to drop a Friendly Boulder when broken. That count doubles in a No Traps world, which is the joke: the seed with no traps in it has the most boulder novelty items. There is no way to tell a marked boulder from an ordinary one on sight, so collecting them means breaking a lot of traps.