Subterranean SecretsCaver Andy Eavis compares his hand size with painted prints on the walls of the recently discovered Black Hands Cave, part of the massive Gunung Mulu cave system in the Malaysian section of the island of Borneo (map).

The handprints are among several finds featured in newly released photographs taken in May during a U.K.-led expedition to the remote caves. In addition to the prints, explorers found prehistoric bacteria alive inside stalactites and an ancient human burial ground perched on a cliff face.

Eavis, a veteran explorer of the Borneo caves, said the handprints have yet to be dated—but he suspects they aren't as old as they seem. Local inhabitants often enter the caves to gather swift nests made from the birds' spit. Considered a delicacy in Asia, particularly China, they are eaten in soup.

"The nests are the same value as silver," Eavis said, and "that cave has quite a bit of modern graffiti from people collecting bird nests."—James Owen(Also read dispatches from a recent conference on prehistoric rock art.)

Photograph by Robbie Shone, Barcroft/Fame Pictures