The Underworld

The Underworld runs the full width of the map along the very bottom, a single enormous open cavern roofed in ash. Lava pools cover much of the ground, Hellstone sits in veins through it, and Ruined Houses stand across the middle stretch. Almost everything you go down for is in one of those three things. The outer sixths are ash forests, the only real vegetation and the calmest part of the layer.

Depth scales with world size. A small world puts the ceiling around 650 tiles down, a medium world around 1,000, and a large world around 1,500. A hellevator, a straight shaft from the surface, is still the fastest way there and doubles as the way back.

Fire protection changes this biome more than damage does. An Obsidian Skull or anything crafted from one lets you stand on Hellstone without burning. An Obsidian Rose cuts lava damage. A Lava Charm buys you a window of immunity when you fall in.

The Terraspark Boots fold most of that into one slot. They let you walk across the surface of lava, give roughly seven seconds of protection if you go under, stop Hellstone burning your feet, and add speed and a short flight on top. The bottleneck is the charm inside them, which now shows up in about 2.5 percent of lower Cavern gold chests and at the same rate in obsidian and hellstone crates. If you want the boots, start fishing crates well before you need them.

Every Hellstone block you break releases about half a tile of lava, so mining a vein from below floods you. Tunnel in horizontally from a sealed pocket, ideally an existing Ruined House, and leave a drain channel to one side so the lava has somewhere to go that is not your feet. Obsidian Platforms let you build over lava without burning away, and a Water Bucket kills a small spill on contact.

Water does not survive down here. Anything you pour in evaporates quickly, so this is a poor place to make obsidian in bulk, though a bucket still handles a puddle. Dumping an ocean through a channel is the exception: enough water crusts the surface of a pool into obsidian before it burns off, sealing the pool permanently.

The Wall spawns whenever a Guide Voodoo Doll touches lava. Voodoo Demons carry those dolls and fly over lava constantly, so a kill in the wrong place starts the fight whether you were ready or not. Minions are the worst offender, because they pick targets without asking. Sealing the lava pools near where you work prevents the accidental summon, stops you falling in, and flattens the ground you will want for the real fight later. It is three problems solved with one job.

For how late it sits in progression, the Underworld is calmer than the Dungeon or the Underground Jungle, simply because enemies appear far less often. That holds into early Hardmode too: nothing new spawns down here until you beat a mechanical boss, at which point Lava Bats and Red Devils arrive. That window is the right time to strip mine.

• Working inside a tunnel keeps most spawns off you, though Fire Imps and Bone Serpents still come through walls of ash.
• Ruined Houses are roomy enough for enemies to spawn inside them. Replacing their background walls stops that.
• Seventy five Meteorite blocks override the native spawn list entirely and replace it with Meteor Heads, which is occasionally the trade you want.

Bone Torches raise your luck down here where ordinary torches do not. In Expert, Lava Slimes leak lava where they die, so the Royal Gel that makes slimes friendly earns its slot on any trip. Minions handle Fire Imps and Red Devils better than most weapons do, and an Imp Staff is the strongest pre-Hardmode option for that, as long as you keep it away from Voodoo Demons.

Because the Underworld spans the entire world, its ceiling is a map. The undersides of the Jungle, the evil biome and later the Hallow are all readable from below, which makes a walk along the bottom the cheapest survey you can run.

Nothing in the Underworld applies background or foreground shading. Light sources work normally, but the usual darkening is absent, so blocks stay lit and the background stays visible no matter what. It is most obvious if you fight the Moon Lord here, where the arena looks nothing like it does anywhere else in the world.