Humans have mapped every corner of the globe–from jungles and deserts to the depths of space. Yet we’ve gone only seven miles below the Earth’s surface–just one five hundredth of the way to the Core. This documentary special goes deeper: 4,000 miles down to the heart of our planet. Along the way, scientists, engineers, explorers and adventurers encounter an underground world where: strange life forms inhabit deadly environments over a mile down; trees force their way through 400 feet of rock to find water; 1500-foot tall skyscrapers are built on sand; mines are the size of cities; and prospectors give the Earth electric shocks to help find oil. It also a place where people run marathons, make parachute jumps and scuba divers edge their way to the base of the world’s deepest caves. Finally, find out how our very existence depends on the mysterious underground forces that shape the Earth.