Every year at Grand Canyon National Park, millions of visitors from all over the world stop at one of a dozen water spigots. Most people are on a rim, seeing the canyon's majesty for the first time, when they step off the trail to refill a water bottle. Others are deep in the belly of the canyon, sweaty and tired, facing a hike up in punishing heat, filling their reservoirs and dumping water over their heads to avoid dehydration and heat stroke.