Karst
Landscape where rainwater slowly dissolves limestone, carving caves, underground rivers and sinkholes, so water flows below ground rather than in rivers.
Karst is the name for landscapes built on rock that dissolves in water, mostly limestone. Rain picks up a little carbon dioxide and becomes a weak acid, which over thousands of years eats channels and caves into the rock.
In karst, water drains underground instead of running off in streams. When a cave roof collapses, it leaves a sinkhole. In Mexico's Yucatán, these water-filled sinkholes are called cenotes, and the state has no surface rivers or lakes at all: its drinking water all comes from the aquifer below.
Karst is common: limestone and similar rocks underlie a large part of the world's land, from Slovenia's Kras plateau, which gave karst its name, to southern China's tower hills.
A common misconception is that underground water in karst is filtered and safe. Because it moves through open channels, pollution from the surface can travel quickly and far, which is a major concern in Yucatán, where homes, farms and hotels all sit above the water people drink.
