Layers

Terraria has four layers. Each layer has its own specific resources and contained environments.

Surface
The surface includes the Forests, Desert, Corruption, Meteorite, and Ocean biomes, as well as the entrance to the Dungeon. This layer has a dirt only backdrop.

Usually spans from +0ft upwards.

Contains:
 * Plant Life - Grass, Trees, Sunflowers, regular Mushrooms and of course Vile Mushrooms in The Corruption.
 * Soil - Dirt, Mud, Sand, Clay, Stone
 * Ore - Iron, Copper, Silver (rare), Gold (rare)
 * Gems - (Very Rare)
 * Water

Underground
The underground contains the Underground, Dungeon, Underground Jungle, and Corruption biomes.

Usually this layer begins at the -0ft mark and expands downward to around -850ft. This layer's backdrop consists of dirt with stones set in it. Starting from this layer downwards, the sky is blocked by an indestructible background which changes at each layer, and just like the sky blocks cannot be placed in the air. However, you are able to place standard destructible backgrounds in front of this permanent background, and the placed backgrounds will act as normal: blocking monster spawns if applicable; allowing the creation of homes if applicable; allowing blocks to be placed in the air.

Contains:
 * Plant Life - Glowing Mushrooms (rare) and Mushroom Grass Seeds (Very Rare)
 * Soil - Dirt, Mud, Sand, Clay, Stone
 * Ore - Iron, Copper, Silver (uncommon), Gold (uncommon)
 * Gems - (Rare)
 * Water

Caverns
The cavern layer contains the Cavern Layer, Dungeon, and Underground Jungle biomes.

Usually this layer begins at the -850ft mark and expands downward to around -3000ft. This layer's backdrop is the same as the Underground, except the dirt is of a darker hue.

Contains:
 * Plant Life - Glowing Mushrooms (rare) and Mushroom Grass Seeds (Very Rare)
 * Soil - Dirt, Mud, Sand, Clay, Stone
 * Ore - Iron, Copper, Silver, Gold
 * Gems
 * Water & Lava

Underworld
The Underworld contains the Underworld biome. Usually this layer begins at the -3100ft mark and expands downward to the bottom of the map. The backdrop looks like dark rock with veins of lava flowing through it.

Contains:
 * Plant Life - None
 * Soil - Only Ash
 * Ore - Only Hellstone
 * Gems - None
 * Lava