Sand. It's one of the world's most vital commodities, though we don't give it much credit. Our cities are essentially made of sand, from the asphalt roads to the concrete buildings to the glass windows. Human beings use more of the stuff than nearly any other resource. And as the global population expands and the world becomes more urban, it is in ever greater demand. Sand mafias in India have reportedly killed hundreds of people, and sand mines are wreaking environmental havoc all over the world. How can we possibly be running low on sand? And what does it mean for the future of civilization? The global sand shortage. Our guest, journalist Vince Beiser, wrote this piece for The New York Times and this one for Wired.