Lake Mead is the largest reservoir in the United States, a huge man-made body of water formed by the construction of the Hoover Dam in the early 1930s. Its 640 square-kilometer surface stores water for tens of millions of people and countless acres of farmland in the southwest.

But it's shrinking at a terrifying rate and now stands at just one-quarter full. The National Park Service, which manages access to the lake, has spent more than $40 million since 2010 trying to keep the water open to boaters. It costs up to $3 million to reconfigure the boat launch ramp every time the water level falls another 120 centimeters.