Two geologists walk into a bar. The first one says to the bartender, “You know, soon there’s going to be a shortage of sand.” The bartender looks up, waiting for the punchline. The second geologist says, “He’s not kidding and that’s not a joke.”

Conjure up images of the Sahara, Gobi deserts or the dunes of Tottori — there are plenty that work; roughly one-third of the Earth’s surface is officially classified as desert, much of it sandy — and it’s hard to believe that worries about a sand shortage aren’t an inside joke among environment researchers.

The cause of the concern is sand is the second-most consumed natural resource in the world, surpassed only by water. It is estimated — and the numbers are very rough — that some 50 billion tons of aggregate (the industry term for sand, gravel and other materials used in construction) are consumed each year for construction. That’s enough to build a wall 27 meters high and 27 meters wide around the world at the equator.