Caves & Karst.

Limestone's own page already mentions karst landscapes in passing, and it's a full glossary term — but the actual story of how a solid rock ends up riddled with caves never gets told. It's a natural, visually dramatic extension of the Limestone page, the same way volcanic islands extended Volcanism.

It starts with the same chemistry already covered on the Limestone page: limestone is made of calcite, and calcite reacts with acid. Rainwater, as it falls through the air and then seeps through soil, picks up dissolved carbon dioxide and becomes very slightly acidic — not acidic enough to notice, but acidic enough that, given enough time, it slowly dissolves limestone as it percolates downward through cracks and joints in the rock.

Over thousands to millions of years, that slow dissolution widens tiny cracks into larger channels, channels into passages, and passages into full caves — an entire underground drainage system carved out of solid rock, purely by mildly acidic water finding its way downward. A landscape shaped by this process is called karst, and it has a recognizable set of features beyond just caves: sinkholes, where the roof of an underground cavity collapses; disappearing streams, which vanish into a crack and reappear miles away out of a spring; and very little surface drainage overall, since most of the water is moving underground instead of in visible streams.

Inside a cave, that same dissolved calcite can come back out of solution and redeposit as dripping water evaporates, slowly building stalactites (hanging from the ceiling) and stalagmites (growing up from the floor) — mineral formations built one microscopic layer at a time, sometimes over tens of thousands of years, from the exact same rock chemistry already explained on the Limestone page, just running in reverse.