Talk:Underground Corruption/@comment-2.26.206.250-20130525122300/@comment-8625790-20130609235543

Obviously you're new to the game. Those are Demon Alters. You can craft boss-summoning items at them, like a crafting table. Before you beat the Wall of Flesh (third boss), that's the only purpose they have. The Wall of Flesh (WoF) drops the Pwnhammer, which is the tool you use to break them. If you try to break them with anything else, it'll take away half your health. If you use the Pwnhammer to break them after neataing the WoF, they will make new, higher tier ores and scatter them randomly in your world. When you break one, a Wraith will spawn, so don't break a bunch all at once. You don't have to worry much about this since you're obviously not very far in the game yet, but I hope this helps and have a fun time playing Terraria!