40 Million People Rely on the Colorado River. It’s Drying Up Fast.

On a 110-degree day several years ago, surrounded by piles of sand and rock in the desert outside of Las Vegas, I stepped into a yellow cage large enough to fit three standing adults and was lowered 600 feet through a black hole into the ground. There, at the bottom, amid pooling water and dripping rock, was an enormous machine driving a cone-shaped drill bit into the earth. The machine was carving a cavernous, 3-mile tunnel beneath the bottom of the nation’s largest freshwater reservoir, Lake Mead.

Lake Mead, a...