Hybrid Serpent people who burrow in Underground Caverns.

"Another mutation of the saurian or serpent race that is capable of burrowing through the earth. Possibly quadrupedal as well as be-pedal, these have been known to use their natural 'boring' abilities to create artificial tunnels like moles, or even spontaneously produced 'cave-ins' (the latter has allegedly been used in attempts to entrap or kill unsuspecting intruders into the underground domains). These may possess a highly- developed 'bio-sensing' system."
http://www.tarrdaniel.com/documents/Ufology/alien_species.html

Disclaimer: please view this material as exploratory, even metaphoric, if you like.