Not of This Earth — Michael Cole

Alexis Brooke, an expert at identifying Mesoamerican structures, is called upon to head up an expedition to the Yucatan Peninsula to investigate a half-buried Mayan ruin and a metallic anomaly that purportedly lies beneath it. 

With the help of three other scientists, she soon discovers that not only is the ruin hollow, but that it also emits strange sounds, sounds that can only be heard at night. When a member of her team discovers a way to open it, she lowers herself into the ancient structure.

But it's not the ruin that puzzles her--it's what lies directly beneath it. Her archeological training doesn't begin to prepare her and her team for what they will find. . . .