A cave explorer stands before an imposing stalagmite made of mineral deposits near 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 spelunking team 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.

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Scot Hoffman
—Photograph by Dave Bunnell