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

Two members of a British expedition stand at the entrance to Racer Cave, part of the Gunung Mulu cave network, which extends underground for hundreds of miles.

Strange, warped stalactites (pictured) hanging from the cave's ceiling are products of bacteria living on the limestone formations, said Eavis, chair of the British Caving Association.

The bacteria's growth is influenced by light from the cave entrance. In turn, the stalactites' growth is influenced by the bacteria, around which mineral deposits collect, resulting in the rock formations' uneven appearance.

Cavers pass surreally shaped pale limestone formations inside Black Hands Cave.

In another part of the vast cave system (not pictured), the U.K.-led team found what is likely an ancient and untouched burial site.

The subterranean cemetery contains human bones and large pottery vessels arranged on an elaborate wooden platform, Eavis said. The site has yet to be studied, but Eavis estimates it to be "anywhere between 500 and 5,000 years old."

The expedition team did not interfere with the remains, taking only photographs. But the photos and the exact location of the unnamed cave are being kept secret to protect the site.

This photo of the north entrance of Deer Cave—until recently the largest known cave passage in the world—is taken from the spot where the first team to explore the Gunung Mulu system camped in 1978. Since then, more than 186 miles (300 kilometers) of caves have been mapped in the area.

Covering 1.8 miles (3 kilometers) of unexplored caves, the recent expedition focused mainly on collecting rock deposits for scientific analysis.

Samples taken include pieces of stalactites with 60,000-year-old rainwater and ancient forms of bacteria trapped inside, according to team member Eavis.

"You've got Jurassic Park bacteria which is still living!" he said. "There are billions of different types of bacteria in the caves."

Measuring some 656 feet (200 meters) high and 492 feet (150 meters) wide in places, Deer Cave (pictured above in May) is home to more than five million bats, which deposit half a ton of guano a day, Eavis said.

(See a picture of a thumb-size bat found in a lava tunnel.)

Such caves are formed by a faintly acidic solution of rainwater and carbon dioxide, derived mainly from the soil, that over time dissolves the limestone rock underneath.

Gunung Mulu has the perfect cave-forming combination of pure limestone, high rainfall, and carbon-rich tropical forests: "Put all that together and you get [some of] the biggest caves in the world," Eavis said.

Looking like an ornamental water feature, a showerhead formation sprinkles near team member Gina Moseley. Described as a "very, very unusual" find by fellow caver Eavis, cave showerheads continue to baffle geologists, who are unsure how such features form.

In general, limestone deposits in the Borneo cave system date back three million years, and they act as a "repository of the history of the planet," Eavis said.

For instance, chemical traces in deposits less than 350,000 years old "can tell you what was going on with the climate with a tremendous degree of accuracy," he said. And ancient pollen traces "will tell you all the vegetation that was growing in the area at the time."

Cavers stand beneath the southern entrance of Deer Cave during the May expedition.

The latest trip to the cave system was part of the Mulu Caves Project, which has been mapping and collecting data on Gunung Mulu since 1978. Plans for 2011 include taking precise laser measurements of Sarawak Chamber, the world's largest cave chamber.

Previous measurements put the chamber at about 390 million cubic feet (11 million cubic meters)—"It's the largest enclosed space on the planet," Eavis said.