Minotaur — Susan Hart

Minotaur, is about an archaeologist hired by a mysterious Greek billionaire as a spelunker to descend into one of the very deep and dark places on his property. There are mysterious secrets, thousands of years old, buried there as they go along a narrow passageway her companion, Mithros, begins to hum a mysterious song; partly in Greek; partly in Latin, and the rest in an unknown to her language which sounds ancient. She loses him in the darkness behind her until she senses some large creature in the dark. Only later, after an incredible encounter, does she learn of the connection between the creature and her employer Mithros, and the true purpose of their adventure into the depths of Crete.