Cavern

The Cavern layer is below the Underground layer and above the Underworld layer. The depth at which the cavern layer starts can be found in the table below. It can be noticed by the darker stone backdrop wall (but it really begins before that background appears). Stone largely replaces dirt in this layer. The Cavern layer is rich in mineral and gem deposits; generally, the deeper one travels, the larger the veins.

Worlds generated after 1.1 patch have here traps, which you have to watch out for.

Lava begins to appear around -2100ft in small pools, which gradually become lava wells at around the -3000ft mark. One must be extra cautious at the lowest levels of the layer at the border between Cavern and Underworld because Underworld enemies can ambush unsuspecting players.

As of version 1.1, it is possible for the Corruption and the Hallow to spread into the Cavern layer when the world is in Hardmode, though this does not stop ordinary Cavern enemies from spawning, but add themed enemies.