Part of the natural backdrop of the world, the earth backdrop is the layer you see below ground after you mine earth blocks. This backdrop looks different depending on what biome you're in. On temperate worlds, for example, earth background is a light yellow-brown color. On hell worlds, it's red with glowy yellow flecks. In deep biomes, it's brown, with a layered, slate-like appearance. On desert worlds, it's a creamy beige, and in brain biomes it's swirly black and brown with light-colored streaks.
How to Obtain
The earth backdrop cannot be mined or crafted, although it can be excavated with the butler bot except in areas close to sea level. Excavated earth backdrop, however, does not go into inventory. In other words, this block is not intended to be obtainable by players.
When onyx was first added into the loot drop rotation in late 2012, a strange bug resulted in earth backdrop blocks randomly dropping as loot along with (or instead of) onyx. The bug was quickly resolved, but some player inventories displayed earth backdrop blocks as obtained items. Shortly thereafter, the developers redesigned the inventory system, making it so players could no longer view these and other glitched items. According to developers, such items do still exist in a few players' inventories, despite being invisible.