Giant Worm

The Giant Worm tunnels through the Underground and Cavern Layer in search of it's prey. It'll pop out at spots in the ground near you, but you'll hear it, so be careful. These things get annoying fast, so aim for the head for a quick kill. Giant Worms follow the Worm AI.

When you hit their head with a pickaxe it's always a one hit kill; so the best thing to do when one spawns near you is just to keep mining untill you hit his head.