Layers

Terraria has five layers of each world. Each layer has its own specific resources as well as contained environments.

Space
Space contains the higher ones of a world’s floating islands.

It covers the top of the world and has distinct music, but no backdrop. Its background is black and star-studded.

The only difference from the surface layer, apart from the background, is the reduced gravity, making players jumping and falling at this height descend slower like under the effect of a featherfall potion.

Enemies are the same as generally in high altitudes, i.e. harpies and wyverns.

Surface
The surface includes the Forests, Desert, Corruption, Crimson, Meteorite, Surface Jungle, Snow 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 Vile Mushrooms in The Corruption.
 * Soil - Dirt, Mud, Sand, Clay, Stone
 * Ore - Iron, Copper or Tin, Silver or Tungsten (rare), Gold or Platinum (rare)
 * Gems - (Very Rare)
 * Water

Underground
The underground contains the Underground, Dungeon, Underground Jungle, Corruption and Crimson, Underground Snow 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. 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 or Tin, Silver or Tungsten (uncommon), Gold or Platinum (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, Silt
 * Ore - Iron, Copper or Tin, Silver or Tungsten, Gold or Platinum
 * Gems
 * Water & Lava

Underworld
The Underworld contains the Underworld biome. Usually this layer begins at the -3000ft 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 - Fireblossom
 * Soil - Only Ash
 * Ore - Only Hellstone
 * Gems - None
 * Lava (All of the water entering here evaporates at this level.)