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 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. Much of the solid surface consists of Hellstone, 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. If any Water enters the Underworld, it will quickly evaporate, but a water bucket can still be used to quench somewhat-smaller amounts of lava. Consider this if you have a large pool of water close to the Underworld which you don't want to lose.

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 surprise and kill most unprepared players.

The middle two quarters of the Underworld contain several Ruined Houses, which are Obsidian and the less common 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's locked Gold Chests, the Golden Lock Box from Dungeon Crates, or dropped from ), and can contain unique Underworld-only loot.

Tips

 * Protective accessories are a must here. An Obsidian Skull or any of its many upgrades will allow walking on Hellstone;  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 by the time you get one, you likely will have one of the other two items.
 * A player can dig a "hellevator", a vertical tunnel two blocks wide down to the Underworld, being wary of lava along the way or at the bottom. With fall protection such as a Lucky Horseshoe (or any upgrade thereof) or Wings, they can simply fall straight from the surface to the Underworld.  (Occasional platforms will slow down the fall, but will also make getting back up much easier!)   Within the tunnel, the close confines will prevent larger enemies from spawning, some other spawns will be reduced, and the walls will protect from most nearby spawns.  (Fire Imps and other enemies which can attack through blocks will still be an issue.)
 * Other options for avoiding fall damage include:
 * Platforms at intervals close enough to avoid fall damage.
 * Grappling on wall with a Grappling Hook to end their fall.
 * The nervy can end their fall with the second jump of a Cloud in a Bottle or similar, or their various upgrades.
 * Running Rope down the hellevator, which also helps with going back up. Rope Coils will help speed this process up.
 * Another option is to end the Hellevator (or a section of it) with a pool of water (above the Underworld proper) or some Pink Slime Blocks at the bottom.
 * If widened to three or four blocks, with an additional three-block barrier of incorruptible blocks, a hellevator can be part of the player's defense against Hardmode's biome spreading.
 * A quick-and-dirty way to create a wide (but perhaps uneven) hellevator, is to simply make a small hole and drop a few stacks of bombs or dynamite down it. Once it gets deep enough for the explosives to detonate while falling, or if the tunnel drifts to the side, the player will need to go down and work from a lower level.
 * A very effective way to counter Fire Imps and Red Devils is with a minion. An Imp Staff can be crafted out of Hellstone Bars.
 * If the player explores the roof of the Underworld, they may find parts of an Underground Jungle.
 * Despite the sequential boss difficulty, the Underworld is actually much easier to survive in than the Dungeon or Underground Jungle. A player should by all means explore the underworld as soon as they have a pickaxe that can mine hellstone, or even before if they would like to make use of Hellfire Arrows or the Demon Scythe spell, so long as they have sufficient means of avoiding the lava. After the Wall of Flesh is defeated and the world enters hardmode, the Underworld may become one of the safer areas of the world, thanks to the the generally stronger enemies and spreading biomes.
 * One way to avoid most of the Underworld spawns is to stay in confined quarters while mining Hellstone:
 * You will certainly want a minion for this -- a Hornet Staff will do until you have your Imp Staff. Without a minion, you will need to block the imp's missiles until they teleport within reach.
 * The Ruined Houses are large enough to allow for some enemies to spawn inside, but replacing their back walls will squelch that. Tunnels and bunkers will also help.
 * Avoid using Stone Blocks for bunkers or bridges, because the smashing of Demon Altars may convert one or more stone blocks into Ebonstone/Crimstone or Pearlstone. While the native Ash blocks are incorruptible, either contagious biome can spread through a stone bridge or shelter, possibly converting enough blocks to produce spawns from that biome.  These blocks may also be difficult to find when trying to completely purify the world.  In any case, using any sort of Wood or Brick will avoid all problems from biome spreading.
 * The Ruined Houses usually have Hellstone veins butting up against them, and more embedded nearby in the ash. 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 here.
 * You can bring 50 pieces of Meteorite to the Underworld and thus create an artificial Meteor biome. The Meteor Heads will no longer drop loot once Hardmode begins, but may still be easier to deal with than the native spawns.
 * It is possible to flood most of the Underworld, by digging channels to dump underground pools, or even an entire ocean, over the lava. The water will eventually evaporate, but not before it converts the lava surface to obsidian.  Dumping an ocean will thus seal off the lava for that side's entire third of the Underworld (the middle third is blocked by the Ruined Houses); It will take some time for all the water to evaporate, and until it does the area will produce both Underworld and Ocean spawns.

History
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