If water drops from along a line, it may form drapes or curtains of stone. Water oozing from a crack in a side may build a waterfall of stone. Natural openings on the surface that lead to caves are called sinkholes, or swallow holes. Streams sometimes disappear down these holes and flow through the cavern. Rivers may flow from one mountain to another through a series of caves.
Some caverns have sinkholes in their floors.
Water often builds up a rim of dripstone around the edge of the hole. Dripping water often contains dissolved minerals as well as acid. If minerals in the water change, layers of different colors may be formed. Staffa Lava Sea Caves, Lava Caves, and Ice Caves Ocean waves battering against the base of a rock cliff will open cracks in weak places. In time the cracks become tunnels, and the tunnels enlarge to become sea caves.