Chthonic

Chthonic is an adjective that means concerning, belonging to or inhabiting the underworld. 

Our word of the day comes from a term in Greek mythology to describe the underworld. By the late 19th century, chthonic had risen from the depths of Greek mythology to inhabit English. Here’s an example:

My son loves reading about Greek myths, especially the really creepy ones about chthonic activity from down below.