MOST Westerners facing criminal charges in Cambodia would be thanking their lucky stars at finding themselves safe in another country. But Alejandro Gonzalez-Davidson, who is half British and half Spanish, is pleading with the Phnom Penh government to allow him back to stand trial along with three Cambodian colleagues. They’ve been charged, essentially, with interfering with the harvesting of one of the 21st century’s most valuable resources: sand.

Believe it or not, we use more of this natural resource than any other except water and air. Sand is the thing modern cities are made of. Pretty much every apartment block, office tower and shopping mall from Beijing to Lagos, Nigeria, is made at least partly with concrete, which is basically just sand and gravel stuck together with cement. Every yard of asphalt road that connects all those buildings is also made with sand. So is every window in every one of those buildings.

Sand is the essential ingredient that makes modern life possible. And we are starting to run out.

That’s mainly because the number and size of cities is exploding, especially in the developing world. Every year there are more people on the planet, and every year more of them move to cities. Since 1950, the world’s urban population has ballooned to over 3.9 billion from 746 million.

According to the United Nations Environment Program, in 2012 alone the world used enough concrete to build a wall 89 feet high and 89 feet wide around the Equator. From 2011 to 2013, China used more cement than the United States used in the entire 20th century.