Today, modern Libyans have tapped into the same ground water supply by using pumps to reach deeper than the Garamantes could. But just like their ancient predecessors, they are exploiting a finite resource. At most, it will last only another fifty years. But water in this most inaccessible stage of the water cycle is found in many other places.

 

It's at its most spectacular in Tallahassee in Florida. Here, divers are just beginning to explore a mysterious series of caves, called the Karst System, carved out by ground water over millions of years. This is one of the planet's least known frontiers. When they began, these divers had no idea of the extents of the cave network. To explore these caves, the've  made the longest dives in history, traveling more than seven miles from the cave entrance. They are sometimes underwater for twenty hours at a time.

 

Their efforts have revealed one of the world's largest underwater cave systems. It's part of a huge store of ground water of varying depths that underlies all of Florida and reaches into neighboring states. And it's not just the U.S.A.. There is ground water in the most unexpected places. More than 30% of all the fresh water on Earth is under our feet. Looked at this way, our apparently solid planet is more like a sponge.

 

In our early history, the need for reliable supplies of water led us to rivers and ground water. But as humans spread across the planet, they learned to exploit the vagaries of the water cycle in many different ways.