The ancient amphitheater of Hierapolis sitting above the town of Pamukkale. The entire city is laid out across the plateau on top of the travertine.
They are white calcite formations caused by water and calcium that builds shelves the way stalactites and stalagmites are made in caves. Pamukkale has the large example of them in the world. There are also some near Oaxaca Mexico that are just as impressive.
-Will
