Oasis

The pool, the palms and the dragonflies look like their own thing, and every check the game runs disagrees. Resident biome preferences, enemy and critter spawns and the fishing table all read it as plain Desert. That cuts both ways. You lose any hope of it counting for something separate, and you gain the ability to build one: put a body of water on the surface of a desert and it behaves as an oasis for every practical purpose.

This is where Oyster come from, and oysters are where Pearls come from, which is the entire Luck Potion chain. Open them for the pearl and treat the Shucked Oyster as a byproduct, because the fish you pull here give a stronger food buff than the oyster does. The crates are the other draw. Both these and Mirage Crate can contain the Magic Conch, and a permanent one-way trip to the Ocean saves a genuinely large amount of walking across a full run.

A hallowed oasis keeps working. Fishing one can produce either the hallowed catches or the oasis ones, so the conversion costs you nothing. Corruption and Crimson do not extend the same courtesy: they overwrite the pool rather than sharing it, so protect a desert you intend to fish.

Larger worlds allow more of them, up to a handful on the biggest, but nothing guarantees even one and plenty of worlds generate without any. A single desert can also produce two. If your desert sits over an Underground Desert, other surface structures compete for the same space and often win, which is the usual reason a promising looking desert has no water in it.

The turquoise is the same shade the surface desert uses elsewhere, and it is not tied to the location. An Oasis Water Fountain reproduces it anywhere you want it, which is the practical way to get that look into a built base. The animal skulls scattered in the background are decoration only, and once you break one it does not come back.