Underground Desert

The Underground Desert is a honeycomb, not a cave system. It is one round or oval mass of sandstone and hardened sand riddled with small chambers that connect to each other at random, and it is large enough to compete with the dungeon for the biggest structure in the world. There is no main corridor and no obvious direction of travel, so the usual habit of following a tunnel and seeing where it goes does not work. You pick a heading and break through walls.

The desert above it usually advertises itself. Roughly half of worlds generate a dedicated entrance structure on the surface, and a pyramid's shaft often punches through the ceiling of the biome on its own, which is the fastest way down when one exists. Neither an oasis nor a pyramid is proof, since both can appear in any sandy patch the world happens to have. The placement rule is more reliable: this pair sits on the same side as the Underground Jungle, opposite the snow biome and the dungeon, with the single exception that it can share a side with the dungeon when generation runs out of room.

Watch the floor near the bottom. The boundary between this biome and the cavern layer holds lava pools much higher than lava normally generates, so ground that reads as a safe depth is not.

This one mechanic explains most of the biome's strange behavior. You count as being in an Underground Desert when you stand in front of unsafe Sandstone Walls or Hardened Sand Walls with enough sand-family blocks around you, the same fifteen hundred tile figure the surface biome uses. The enemies, and the bound Golfer who spawns down here, do not check the biome at all. They check the walls. That is why desert enemies turn up where nothing looks like a desert, and why a room you seal with hand-placed sandstone wall goes silent: placed walls count as safe and shut spawns off.

The game picks a candidate tile and then tests it. The tile has to be a solid active block that has not been actuated, platforms included, it has to sit lower than eighty tiles above the surface line, and it has to be past the halfway mark between the surface and the world ceiling.

Above the cavern layer there is a one in three chance the game runs a wide check rather than a narrow one. It rolls a number from five to fourteen and scans a square centered on the candidate tile, between eleven and twenty-nine tiles on a side, and one applicable wall anywhere inside that square is enough. When the roll fails it only looks at the wall directly behind your own hitbox. Deeper down it falls back to the wall behind the candidate tile or the tile above it. So a small patch of unsafe wall well away from you can still feed spawns, and clearing walls around a farm matters more than clearing blocks.

• Ghoul variants each need the matching pure, corrupt, crimson or hallowed biome.
• Light Lamia and Sand Poachers need pure or hallowed ground.
• Dark Lamia and Desert Spirits need corrupt or crimson ground.
• Antlion Larvae only come out of broken Antlion Eggs.

The reason this place mauls fresh characters is knockback, or the lack of it. Antlion Chargers and Swarmers move fast and shrug off hits that would stop anything on the surface, and Rolling Cacti come at you along the floor from outside your light radius. Gold or platinum armor is the honest minimum. Grenades do more work than any early weapon here because they clear a chamber you have not opened yet, and whatever movement accessory you own should be equipped before you go in.

Minions are unusually good here even if you have no interest in playing summoner. You can put one on the far side of a wall, it reacts to something fast without you having to aim, it applies knockback while you are busy running, and it will set off a rolling cactus from a safe distance. That last trick takes a couple of attempts, since you may have to summon directly onto the cactus.

Sand, hardened sand and sandstone all convert, which makes this one of the easiest biomes in the world for corruption, crimson or hallow to eat. The existing enemies get stronger when it happens, and a corrupt or crimson conversion adds Desert Spirits. Unlike most infected underground biomes, it does not import that biome's ordinary underground roster, so what you end up with is a harder desert rather than a mixed one.

You need unsafe walls, and the placed versions of Sandstone Wall and Hardened Sand Wall are safe. Treacherous Hardened Sand Walls and Treacherous Sandstone Walls place the unsafe versions directly, and they come out of shimmer transmutation. The other route is conversion: a Clentaminator or Terraformer turns safe walls into unsafe ones, so Blue Solution on a safe sandstone wall gives you unsafe pearlsandstone, yellow works on plain stone, Green Solution works on safe hardened ebonsand, and whatever wall you already have can usually get there by some route.