The Underworld

The Underworld (also known as Hell) is the deepest layer of the world, marked by dark rock with orange lines through it near the top followed by a spacious cavern. It is a large, 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 numerous Ruined Houses, which are natural structures containing unique furniture and other loot. The Wall of Flesh boss needs to be summoned in the Underworld; defeating it will cause the world to convert to Hardmode.

The Underworld contains many hazards, most prominently its broad expanses of lava and several unique and difficult enemies. Touching Hellstone, the valuable crafting material only obtainable there, without proper equipment inflicts the Burning debuff and mining it releases lava. Furthermore, because the Wall of Flesh is summoned by dropping a Guide Voodoo Doll into lava, killing a Voodoo Demon positioned over lava can inadvertently summon it.

The middle two quarters of the Underworld contain several Ruined Houses, which are towers made from Obsidian Bricks and, less commonly, Hellstone Bricks that contain Shadow Chests, Hellforges, Pots, Obsidian furniture, and other lootable items. Shadow Chests require the Shadow Key to open (acquired in the Dungeon's Locked Gold Chests, Golden Lock Boxs, or dropped from Arch Wyverns ) and contain unique Underworld-only loot.

Tips

 * It is highly recommended to equip protective accessories when visiting the Underworld. An Obsidian Skull or any of its upgrades will allow walking on Hellstone and Hellstone Bricks and a Lava Charm or any of its upgrades will protect from brief contact with lava. An Obsidian Rose can also reduce the damage from lava, but due to its low drop chance, one of the other two items is most likely obtained first.
 * A very common method to quickly travel to the Underworld is to dig a hellevator, a vertical shaft from the surface to the Underworld.
 * Minions are a very effective way to counter Fire Imps and Red Devils. An Imp Staff is the best choice pre-Hardmode. Be careful not to let the minions kill any Voodoo Demons above lava pools, as a dropped Guide Voodoo Doll touching lava causes the Guide to die and the Wall of Flesh to spawn.
 * Since the Underworld spans the entire length of the world, it is easy to locate the Underground Jungle, Corruption/Crimson, and Hallow biomes. The bottoms of these biomes are often seen at the roof of the Underworld.
 * Despite the sequential boss difficulty, it is actually much easier to survive in the Underworld than in the Dungeon or Underground Jungle due to the much slower spawn rate. In contrast to those biomes, the Underworld can be explored fairly safely. When starting Hardmode, the Underworld is one of the safest areas of the world because there are no new or more difficult enemies spawning there until defeating a mechanical boss causes Lava Bats and Red Devils to spawn.
 * One way to avoid most of the Underworld spawns (except Fire Imps and Bone Serpents) is to stay in confined quarters while mining Hellstone. Ruined Houses are usually a safe base to tunnel to Hellstone veins from. With care, a player can tunnel directly from shelter through the ash, leaving spaces for lava to drain away from the player as they mine. Obsidian Platforms and Water Buckets will be helpful for this method.
 * The Ruined Houses are large enough to allow some enemies to spawn inside, but replacing their background walls will prevent that. Tunnels and bunkers will also help.
 * It is very helpful to seal all lava pools, because that prevents both inadvertent Wall of Flesh summonings and accidentally falling into a pool. Furthermore, it serves as terrain preparation for the Wall of Flesh fight.
 * Avoid using Stone Blocks for bunkers or bridges, because the smashing of Altars may convert one or more Stone Blocks into Ebonstone/Crimstone or Pearlstone. While these biomes do not usually appear in the Underworld and cannot infect the usual blocks there, introduced stone bridges or shelters can spread the contagious biome following the block conversion. If enough stone blocks were placed, it might possibly convert enough of them to produce spawns from that biome. These stone blocks may also be difficult to find when trying to completely purify the world. By contrast the native Ash Blocks are incorruptible and are suitable for bridges, or in any case, processed Stone Slabs from the Heavy Work Bench or using any sort of Wood or brick will avoid all problems from biome spreading.
 * The native Underworld spawns can be overridden by placing at least 75 blocks of Meteorite, effectively creating an artificial Meteorite biome.
 * It is possible to flood parts of the Underworld by digging channels to dump an ocean over the lava. The water will eventually evaporate, but not before it converts the lava surface to Obsidian, sealing off many lava pools.

Trivia

 * When the player gets close to the Underworld, the screen will start to waver slightly, in a manner similar to the shimmering caused by heat.
 * In the, there is an icon of the power-up from the 1990 side-scrolling platform game  in the bottom-right corner of the Underworld.
 * Just above the Underworld (if backgrounds are switched on), there will be the old background that the Underworld used to have.

History
Unterwelt Enfer 地狱