Underground Jungles are exactly as the name suggests; jungles that are found underground. What the name doesn't tell you is that they feature the dangerous Quicksand, as well as a host of new trees and deadly mobs.
Flora is incredibly abundant in these biomes, featuring an astounding five species of tree, including two exclusive species! Those trees are vanilla Jungle Trees (Cocoa Beans and all!), Asper Trees, Eucalyptus Trees, and the exclusive Mossbark and Mahogany Trees.
Underground Jungles aren't without their own dangers, however; as mentioned before, there are random clumps of Quicksand around, as well as roof-spawned Wasp Nests. On the less dangerous side, there are clumps of Amber and interspersed lakes with Mud lakebeds, often with Bullrush growing alongside.
Many a mob calls the jungles of the Erebus home: