Underground Corruption

The Underground Corruption is the same as the corruption, but at a depth just below where the Shadow Orbs spawn in The Corruption, it has even more dangerous enemies, suchas the World Feeder, Clinger, Slimer, Corrupted Slime and Slimelings, which spwan in sets of 3 upon a Corrupted Slime's death.

it is particularly small until the defeatism of the Wall Of Flesh,then it allows all the corrupted stone or Ebonstone to spread.the only way to stop it is to block it with The Hallow biome.