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 contains 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 Boxes, or dropped from Arch Wyverns) and contain unique Underworld-only loot.
- Breaker Blade
- Clockwork Assault Rifle
- Laser Rifle
- Warrior Emblem
- Ranger Emblem
- Sorcerer Emblem
- Summoner Emblem
- Badger's Hat(when defeated on the same day
as the Eye of Cthulhu)
- Wall of Flesh Mask
- Wall of Flesh Trophy
- Treasure Bag(drops the Demon Heart in Expert Mode)
- Goat Skull(1/4 drop in Master Mode)
- Wall of Flesh Relic(in Master Mode)
NPC happiness 
|Underground / Cavern / The Underworld|
For more information, see NPC happiness.
- The Underworld starts at about 3000 feet (1500 tiles) underground in a large world, at about 2000 feet (1000 tiles) underground in a medium world, and at about 1300 feet (650 tiles) underground in a small world.
- If any Water enters the Underworld, it will quickly evaporate. Therefore, attempting to generate Obsidian here is usually not very effective, due to most of the Water evaporating before touching any lava. However, a Water Bucket can still be used to quench somewhat smaller amounts of Lava.
- On the Desktop version, Console version, Mobile version, and version, the music track The Underworld plays ( "Hell" Music Box) in the Underworld. On the Old-gen console version, the music track Eerie plays.
- No background or foreground shaders can be applied by anything in the Underworld aside from normal lighting devices. This is apparent when fighting the Moon Lord here, as blocks are not darkened, and the background is still visible, unlike in the rest of the world.
- On Old-gen console version, the lava flowing through the cracks of the walls won't appear if a player uses any weapon that produces light on its own but if the weapon used is a sword that is able to shoot a beam like the Frostbrand, the beam will make the lava appear in the cracks.
- 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.
- An Obsidian Rose can also reduce the damage from Lava.
- A Lava Charm or any of its upgrades will protect from brief contact with Lava, and can also be combined with other accessories, culminating in the Terraspark Boots. Note that in Desktop version and Mobile version, these charms have become much rarer, appearing solely in 2.5% of lower-Cavern Gold Chests, and 2.5% of Obsidian and Hellstone Crates.
- On the Desktop version and Mobile version, the Terraspark Boots represent a nearly sovereign remedy for the hazards of Lava. While they don't provide complete protection against swimming in Lava, they allow walking on its surface without harm, and they do offer 7 seconds of protection (extendable by other accessories) for a player who is accidentally dunked. They also protect the player from taking damage when walking on Hellstone or Hellstone Bricks. (Or Meteorite, but that's rare in the Underworld.) Combining all this with their speed and limited flight, they allow moving around the Underworld with fair impunity.
- 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 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 beneficial 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 convert enough of them to produce spawns from that biome. These stone blocks may also be difficult to find when trying to purify the world completely. 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.
- Luck can be increased in the Underworld by using Bone Torches.
- In Expert Mode, killed Lava Slimes will cause Lava to pour out from where they die, so it is recommended to have the Royal Gel accessory equipped as it will make Lava Slimes friendly.
- When the player gets close to the Underworld, the screen will start to waver slightly, like the shimmering caused by heat.
- In the version, there is an icon of the fire flower power-up from the 1990 side-scrolling platform game Super Mario World 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.
- There was originally going to be a "Hell alternative" that would have had the color schemes from hell in Disney's Hercules. However, the idea was scrapped.
This content is transcluded from Biome backgrounds § Underworld backgrounds.
Unlike the underground backgrounds, these have multiple layers like surface backgrounds do.
- Desktop 126.96.36.199:
- Added Otherworldly Underworld track from the cancelled Terraria: Otherworld.
- Added two new backgrounds as well as the one introduced in 188.8.131.52 being now animated.
- Added Hell Butterfly, Lavafly and Magma Snail critters.
- Added two ambience objects in the background: flocks of Hellbats and a Bone Serpent leaping in Lava.
- Bone Serpents now drop Bone Serpent Kite during Windy Days.
- Desktop 1.3.3: Added heat distortion effect.
- Desktop 1.2.4: The Underworld (as well as the Dungeon) has more new furniture generated at world creation.
- Desktop 1.2.3: Fixed the bug where removing walls in the Underworld would not update the map.
- Desktop 1.2: Added Obsidian furniture to ruined houses, and allowed rooms to generate horizontally. Also added Red Devil, Lava Bat, and new backgrounds.
- Desktop 1.0.6:
- Desktop 1.0.5:
- Desktop-Release: Introduced.
- Console-Release: Introduced.
- Switch 1.0.711.6: Introduced.
- Mobile-Release: Introduced.
- 3DS-Release: Introduced.
- Terraria's 8th Anniversary - Ask Redigit and Cenx! May 20, 2019