To build those cities, people are pulling untold amounts of sand out of the ground. Usable sand is a finite resource. Desert sand, shaped more by wind than by water, generally doesn’t work for construction. To get the sand we need, we are stripping riverbeds, floodplains and beaches.

Extracting the stuff is an estimated $70 billion industry. It runs the gamut from multinational companies’ deploying enormous dredges to villagers toting shovels and buckets. In places where onshore sources have been exhausted, sand miners are turning to the seas…

We can make more sand, but crushing rock or pulverizing concrete is costly, and the resulting sand is ill suited for many applications. We can use alternative substances for some purposes, but what other substance can we possibly find 40 billion tons of, every year?…

It once seemed as if the planet had such boundless supplies of oil, water, trees and land that we didn’t need to worry about them. But of course, we’re learning the hard way that none of those things are infinite, and the price we’ve paid so far for using them is going up fast. We’re having to conserve, reuse, find alternatives for and generally get smarter about how we use those natural resources. That’s how we need to start thinking about sand.