Geyser

The ones you find underground generate bare, with no plate and no wire attached, so mining one out gives you a clean trap with nothing to untangle. Since it never needs ammunition, it is one of the cheapest things you can build a farm around once you have collected a handful.

It fires straight up or straight down and nothing else, so it wants a shaft rather than a room. The useful quirk is that the burst will tunnel through solid blocks and liquid looking for the first open space, which lets you bury the trap out of sight and still have fire appear where enemies walk. Stack enough material above it and nothing comes out at all, so test the placement before you seal it in.

The gap between shots is long enough that a single geyser will not hold a corridor on its own. The setup that works is several of them spaced along the floor, all wired to one timer, so something is always firing while the rest recharge. They light the area heavily while they do it, which is either free lighting or a nuisance depending on the build.

Dangersense marks them, which matters because the natural ones now trigger on contact rather than waiting for a wire. Running through the deep cavern layer without that buff is how people lose a character to a trap they never saw.