The word "chthon" is a rare and somewhat obscure word, often associated with ancient mythology and the underworld. However, there are several synonyms that can be used in its place. A few options include "subterranean," which refers to something beneath the earth's surface, "infernal," which suggests a connection to hell or the devil, and "subaqueous," which refers specifically to things beneath the water's surface. Other possible synonyms include "nether," "sublunary," and "subterrestrial." Ultimately, the choice of synonym will depend on the context and the specific connotations desired.