It is one of Earth's few landforms visible from space, a World Heritage site, and America's most popular National Park - with over five million visitors a year. But at the edge of a chasm one billion times their size, visitors see only a tiny fraction of the Grand Canyon's true beauty hidden amidst its stone, its water and its wildlife. It's an undisputed heavyweight wonder of the world, but only around 2% of the Grand Canyon's 1.2 million acres is regularly visited by humans. Some of its backcountry has almost certainly never felt the tread of a human foot. Scientists know precious little about the Canyon's wildlife, its vegetation - even how and when it was formed. Now, a rare scientific odyssey is about embark into this Great Unknown - to explore its entire 277-mile length.

Originally released by National Geographic, 2009. Streaming video file encoded with permission for digital streaming by Infobase on April 07, 2015.

Scientists and the Grand Canyon (6:04); --Rapids Provide Clues (5:31); -- Grand Canyon Dam (3:06); -- Direction of the Flow of the Colorado River (5:21); -- First Humans in the Grand Canyon (9:04); -- Ecosystems of the Grand Canyon (3:53); -- Condor and Mountain Lion (12:19); -- The Great Unconformity (4:38); .