A maw is a hole in the background of a world through which various hostile mobs, or even poison gas (in deep biomes), may emerge.

Placing a plug over a maw will close it off so mobs can't escape. Plugging a maw earns 5 XP.

If you are unable to craft plugs to seal off a maw, you can place blocks around it to trap hostile mobs.

Maw Types

Common Maws resemble cracked gaps in the background and typically spawn terrapus, bats, roaches, or other mildly annoying or hostile creatures.

Culverts are circular maws with a brass ring around the outer edge and typically spawn more dangerous mobs, such as Automata and Brains.

In the Deep Biome, culverts explode, emitting a poisonous gas that is often lethal. The explosions also deal high damage. They cannot be plugged until after the world has been purified.