The water coming out of your kitchen tap is four billion years old and might well have been sipped by a Tyrannosaurus rex. Rather than only three states of water—liquid, ice, and vapor—there is a fourth, “molecular water,” fused into rock 400 miles deep in the Earth, and that’s where most of the planet’s water is found. Unlike most precious resources, water cannot be used up; it can always be made clean enough again to drink—indeed, water can be made so clean that it’s toxic. Water is the most vital substance in our lives but also more amazing and mysterious than we appreciate. ....But the era of easy water is over.