The Underworld

The Underworld (also known as Hell) is the deepest layer of the world, marked by a unique animated background displaying veins of flowing lava, and a second background depicting a large, open space with magma pools and pillars. It is largely an open cavity that stretches across the world's bottom, with a ceiling of Ash Blocks and a lower terrain of large Lava pools, Ash Blocks, and Hellstone. It also contains several Ruined Houses, which are natural structures containing unique furniture and other loot. The Underworld is considered the most challenging biome prior to Hardmode.

The Underworld contains many hazards, most prominently its broad expanses of Lava. It also contains several unique and difficult enemies. Hellstone, of which much of the solid surface consists, is a valuable crafting material that can only be obtained in the Underworld; however it inflicts damage to players on contact without the proper equipment, while mining it also causes more Lava to form. Any Water that enters the Underworld "evaporates" (disappears).

The Wall of Flesh boss can be summoned here. Defeating it will cause the world to convert to Hardmode. It is possible to summon the Wall of Flesh accidentally, by inadvertently killing a Voodoo Demon positioned over lava. This is likely to kill most unprepared players.

The middle two quarters of the Underworld contain several Ruined Houses, which are Obsidian and Hellstone Brick towers that contain Shadow Chests, Hellforges, Pots, Obsidian Furniture, and other lootable items. Shadow Chests require the Shadow Key to open (acquired in the Dungeon), and can contain unique Underworld-only loot.

Tips

 * After hardmode, the Underworld will be, ironically, the safest place after everything is either Corrupt, Jungle, Hallowed or Crimson. However, once a Mechanical boss is defeated, Red Devils and Lava Bats will spawn. (Note: this can happen before hardmode, if a boss is summoned and defeated before the Wall of Flesh.)
 * You can dig a tunnel two blocks wide down to the bottom of the world. With a Lucky Horseshoe, Wings, or by jumping with a Cloud in a Bottle at the end, you can fall down this tunnel, commonly called a "hellevator" (even in other games.) Since you're in a two-block-wide space, monster spawn rates are heavily reduced, especially for the annoying Fire Imps. This gives you the chance to mine as much Hellstone as you want, but use caution when expanding this tunnel; a larger cave has higher chances of Fire Imp spawns.
 * You can bring 50 pieces of Meteorite to the Underworld and thus create an artificial Meteor Biome.
 * In a large world the underworld is about 3000 feet deep; in a medium world it starts around 2000 feet deep.
 * If you stick around the roof in the top part of the Underworld, you may find parts of an Underground Jungle.