In the year 2000, scientists officially confirmed that sand from Santa Teresa in Cuba could be used to create a new physical phenomenon known as a "revolving river." See a demonstration, and learn about the mystery of Cuban sand.
Ernesto Altshuler of the University of Havana noticed, in 1995, that sand poured in a pile would create a sort of "river" of sand sliding down the pile. The river revolved around the pile, building it up steadily. As the pile grew higher the river revolved more intermittently, making waves on the pile. It was an interesting phenomenon, but surely, with something as common as sand, it had already been extensively studied. It wasn't until Altshuler got to Houston that he realized that his sand wasn't quite as common as he had thought it was.
The region of Santa Teresa, in Cuba, produced sand that, for some reason, behaved in a way that no other sand did. Or, at least, it behaved as no other sand yet discovered did. (After Altshuler published a paper on the revolving river sand, more samples were found from around the world, but it's still very rare.) The formation of the river is comparatively simple. As the river flows down one side of the sand pile, it builds up in a "delta." It, essentially, creates a little pile on the side of the main pile of sand. When the delta gets big enough, it forces the sand river to one side, where the process starts all over again. The river revolves, and builds up the pile.
But why? The samples don't appear to have any unusual characteristics. Whatever it is that makes this sand special does have something to do with its external properties, though. Use the sand too much and that property wears away, so a given batch of sand will eventually stop making a revolving river.