I was looking around in the Underground Jungle, and I saw this:
It looks like a pyramid, but it's deep underground! I also remember reading that there can't be a pyramid and a living tree in the same world, and there's a living tree in my world. (Pyramids are supposed to spawn in deserts, too.)
What is this? I died shortly after (I only had an Ice Boomerang, Trident, and Musket to fight with, and Meteor Armor), and it's quite hard to mine out that block and see what it is while being attacked by jungle enemies. It might not even be possible to mine with the Nightmare Pickaxe.