During the first ever scientific expedition into a volcanic magma chamber, climber Einar Stefánsson rappels into Iceland's dormant Thrihnukagigur volcano in October. Magma chambers supply the molten rock that oozes or bursts onto Earth's surface during an eruption.
Thrihnukagigur, which last erupted about 3,000 years ago, contains only ancient magma—though the volcano could come back to life at anytime, experts say. (See "'Sleeping' Volcanoes Can Wake Up Faster Than Thought.")
"Thrihnukagigur is unique. … It's like somebody came and pulled the plug and all the magma ran down out of it," said volcanologist Haraldur Sigurdsson, who appears in Into Iceland's Volcano, a new documentary featuring the expedition, airing Friday on the National Geographic Channel. (The channel is partly owned by the National Geographic Society, which owns National Geographic News.)
Thrihnukagigur is located about a hundred miles (160 kilometers) from Iceland's Eyjafjallajökull volcano, which blew last April and grounded airplanes for several days with its ash clouds.
While people have ventured inside the relatively shallow volcanic craters located close to Earth's surface, the 2010 expedition was the first to explore a volcano's deeper chambers.
"It was a really amazing experience—just unbelievable," Sigurdsson said.
ON TV: Into Iceland's Volcano airs Friday, April 8, at 8 p.m. ET/PT on the National Geographic Channel. Preview >>