Chasms

A chasm is the hole your world evil punches into the map at generation. From the surface it reads as a dark break in the ground. Below that it is a shaft lined with Ebonstone or Crimstone running down to roughly where the Cavern layer starts. Enemies from the biome drift into them and collect at the bottom, so the deepest part of a chasm holds the densest pack of hostile things you will meet in the first hour of a world.

The Corruption version is a straight vertical drop, walled and floored in ebonstone, with an ebonstone background behind it. Near the bottom a horizontal tunnel usually joins several shafts together and carries the Demon Altars.

The Shadow Orbs are not in the shaft. They sit buried some way beneath the floor, sealed inside solid ebonstone, and you will walk over them without noticing unless you are watching the minimap or wearing something that pings treasure. Getting through that seal is the whole difficulty of a first descent.

A Crimson chasm is easier to enter and harder to read. The mouth slopes instead of dropping, and rather than one shaft you get a branching cave that zigzags down into wide chambers shaped like open hands. Each finger is a vein ending at a Crimson Heart, walled in the same way the orbs are. The altars cluster at one depth on the way down rather than in a single corridor.

Those chambers flood often. Carry a Gills Potion or a Flipper Potion on a first trip, or a Breathing Reed if you have nothing better, because drowning in the room with the heart in it is a common way to lose a run.

• Purification Powder from the Dryad. Thrown at ebonstone or crimstone it turns the block back into ordinary stone, which any pickaxe removes. Cheapest route, available the moment she moves in.
• Explosives. Bombs and Dynamite from the Demolitionist ignore block hardness completely. Fast, but they open the chamber and everything next to it, and you will sometimes blow the orb straight into a pit of water.
• The right pickaxe. A Nightmare Pickaxe or Deathbringer Pickaxe cuts ebonstone and crimstone directly. Slightly circular, since the ore comes from the same biome, but by the time you have enough of it you usually do.

Every third orb or heart you smash summons the Eater of Worlds or the Brain of Cthulhu right where you stand, at the bottom of a pit, with no arena and no room to move. Count as you go. If you are not ready, stop at two and break the third somewhere you have prepared. Breaking one also has a chance to drop a Meteorite site into the world, so clearing chasms and hunting meteors are the same errand.

The drops are the reason for all of it: Musket or The Undertaker, Vilethorn or Crimson Rod, Ball O' Hurt or The Rotted Fork, Band of Starpower or Panic Necklace, plus the Aqua Scepter on the Corruption side. The gun and the mana band are the two that visibly change how a run plays.

Early on a Corruption chasm is a trap for one simple reason: you can get down and you cannot get back up. The walls are stone you cannot mine, the ledges are spaced too far to jump, and there is nothing to hook onto. Without a Grappling Hook or a rope you can end up stuck at the bottom until something kills you.

Once you commit, run Rope down the shaft as you descend and lay wood platforms across it every twenty tiles or so. They work as a ladder on the way out and as a brake on the way down. A Lucky Horseshoe off a Floating Island removes the fall damage question entirely.

Near a base, cover the mouths. A single row of platforms across the opening stops you dropping in during a night run and you can still fall through on purpose when you want to.