Karst is a type of landscape formed when slightly acidic water slowly dissolves certain kinds of rock, especially limestone. Over thousands of years, rainwater seeps into cracks in the rock and carves out spectacular underground caves, sinkholes, and disappearing streams.

In karst regions, rivers sometimes vanish into holes in the ground and flow through hidden cave systems before emerging miles away. The ground may suddenly collapse to form bowl-shaped sinkholes, some large enough to damage or destroy buildings. Underground, water hollows out enormous caverns decorated with stalactites and stalagmites.

Mammoth Cave in Kentucky, one of the world's longest known cave systems, formed in a karst landscape. So did many famous caves around the world, from China's Stone Forest to Slovenia's Postojna Cave. About 20% of Earth's land surface is karst terrain.

Geologists use karst to describe both the landscape itself and the process that creates it. If you've ever explored a limestone cave or seen a sinkhole, you've witnessed karst geology in action.