The Underground World - S2.E5

Despite all of our modern technology and sophistication, there is much that we still don't know about what lies beneath our feet, deep underground. What is at the center of the Earth? Is there really a solid core of heavy metals, or could it be a hollow environment, as many researchers claim? And just how much of mankind's history took place underground? Recent discoveries of ancient underground structures such as the city of Derinkuyu and the Malta Hypogeum suggest that there may be more structures down there, waiting to be re-discovered. If much of our past was subterranean, could our future be as well?