Layers

Terraria has many 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, backdrops become indestructable.

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.

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