What is this pyramid-like structure in the Underground Jungle?

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.

Solution 1:

That is a new type of dungeon in the underground jungle. You fight the Golem there at the Lihzahrd Altar.

You can break the blocks once you kill the Golem. You can't kill the Golem till hardmode is activated.

Solution 2:

Like others have said, this is a Jungle Temple. It can be entered by unlocking the door with a Jungle Key, obtained in Hardmode by killing Plantera. The Key itself will always drop when Plantera is killed.

The Lihzard Bricks it is made of, as well as the Lihzard Altar inside, can only be destroyed by a tool with over 200% pickaxe power.

Hence, to relocate the Altar, the Golem must be fought. There is a 12.5% chance (Source: http://terraria.gamepedia.com/Golem ) that the Golem will drop the Picksaw, which has 210% pickaxe power -- the strongest in Terraria and enough to break the Lihzard Altar. This can be beneficial for relocating the Altar to a player-made arena.