Limestone is a common sedimentary rock made mostly of calcium carbonate, the same material found in seashells and coral. It forms over millions of years from the compressed remains of ancient sea creatures whose shells and skeletons settled on ocean floors, layer upon layer, until pressure turned them into solid rock.

You can recognize limestone by its usually light color (white, tan, or gray) and the way it fizzes when you drop vinegar on it. That fizzing happens because the acid in vinegar reacts with the calcium carbonate. Limestone often contains visible fossils of the creatures that became part of the rock.

People have used limestone as a building material for thousands of years. The Egyptian pyramids, many medieval cathedrals, and countless other historic structures were built from limestone blocks. It's still quarried today for construction, making cement, and even as an ingredient in some medicines and toothpaste.

Limestone landscapes create dramatic scenery. When rainwater (which is slightly acidic) dissolves limestone over time, it carves out caves, sinkholes, and underground rivers. Famous cave systems like Mammoth Cave in Kentucky formed this way. Above ground, limestone cliffs and formations appear in places from the White Cliffs of Dover in England to stone forests in China.