Many of the Underplane's inhabitants are descended from top-dwelling creatures, that became lost in the caves and gave up hope of ever returning home. Examples include the Cave Eagle and the Naked Mole Bear.
Other creatures are specially adapted to the area, using such adaptations as sonar and bioluminescence. Many mushrooms give off blue light, to attract insects to eat.
The Underplane has existed for as long as there has been a Mythos.