The Underworld

The Underworld is currently the deepest layer of the World, and is considered the most challenging environment so far. It is marked by a unique animated background that shows veins of flowing lava. The Underworld contains many hazards, including large amounts of Lava and Hellstone, both of which can cause a fairly quick Death on contact to players who are not properly equipped. It also contains unique and highly challenging enemies. The payoff for visiting The Underworld is access to large amounts of Hellstone, the main ingredient of Hellstone Bars, which are required to craft some of the best Armors, Weapons, and Tools.

Some of the most challenging enemies spawn here, including Fire Imps, Hellbats, Bone Serpents, Demons, and Lava Slimes. The Underworld terrain is largely made up of Ash Blocks, Lava pools, Hellstone ore, and numerous Hellstone Brick structures that contain Chests and Hellforges. Fireblossoms sprout from ash blocks here.

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.

As of patch 1.0.5, Water evaporates when it reaches the Underworld layer.

Tips and Tricks

 * You can dig a tunnel two 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 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.