The Underworld

Deep under the Cavern Layer, The Underworld begins. The spawning of Fire Imps and Bone Serpents and a change in background texture marks the beginning of The Underworld. It is made of Ash Blocks, lava, and Hellstone ore. Fireblossoms can be found on ash blocks next to lava pools. There are also Hellstone Brick buildings scattered across the landscape, which sometimes contain Hellforges.

The Underworld is the most challenging environment so far, giving you access to Hellstone, which can be smelted with Obsidian in a Hellforge to create a Hellstone bar, the main ingredient of some of the strongest armors, weapons and tools. Hellstone is not difficult to find (it's everywhere), but it's heavily guarded.

As of patch 1.0.5 water Evaporates when it reaches the Underworld layer. Despite the AI balance of this patch, the difficulty of this biome remains due to how Ash Blocks can potentially crumble and drop lava on you as you are mining. This, in many players' opinions, has changed the main obstacle of The Underworld to its lava, instead of the powerful monsters.

It's worth noting that, if you move down as much as you possibly can, the camera will stop aligning with your character because you reach the very bottom of the world.

The Underworld is found at approximately 1500 ft on a small world, 2500 ft on a medium world, and 3500 ft on a large world.

Tips and Tricks

 * You can dig a tunnel 2 blocks wide down to the bottom of the world which then reduces the monster spawn rates, especially for the annoying Fire Imps. However, this also reduces the chances of Bone Serpents, though if they do attack you try to make them run off the entire map/screen and they'll disappear. This gives you the chance to mine as much Hellstone as you want, but be warned! You need to be careful as you expand your cave; a larger cave has higher chances of Fire Imps.
 * You can bring 50 pieces of Meteorite to the Underworld and thus create an artificial Meteor Biome.
 * Water now cannot be brought down in to The Underworld as of patch 1.0.5.