October 20, 2008A cave explorer stands before an imposing stalagmitemade of mineral depositsnear an entrance to the Xe Bang Fai River cave in central Laos.

An expedition in February 2008, co-led by veteran caver John Pollack, comprehensively mapped and photographed the 5.9-mile (9.5-kilometer) length of the little-known cavern for the first time.

The explorers encountered some of the largest rooms and most impressive structures of any river cave on Earth, Pollack said.

A river cave is any cave with an active water source flowing through it.

Everything about the cave is big-from its towering entrances to its phobia-inducing spiders, which can be 10 inches (25 centimeters) across, Pollack added.

"It's also extremely well decorated with spectacular formations," Pollack said.

Scot Hoffman