The limestone that houses Coronado Cave formed about 250-300 million years ago when southern Arizona was covered by a shallow sea. Limestone is made of calcite (calcium carbonate grains), the remains of ancient corals, sponges, shellfish, and other creatures that use calcium and carbon dioxide dissolved in seawater to make their shells and skeletons. When the creatures die, their remains settle to the sea floor where they are broken into sand and silt particles by waves. These deposits eventually become compacted and hardened into limestones as they are buried by younger formation.
Coronado Cave was formed by water seeping down off the mountains through cracks in the limestone. Caves form near or just above the level where the rocks are filled or saturated with water. Calcite grains in the limestone are slowly dissolved by the water. Even normal rainwater becomes slightly acidic as it absorbs carbon dioxide gas from the air and percolates through soils rich in carbon dioxide released by insects, bacteria, and plant roots. The rough, pitted surface of many flat limestone slabs exposed to rainwater along the cave trail is evidence of this process. Another source of a much stronger acid is iron sulfide. Its minerals (primarily pyrite) are in the rocks surrounding Coronado Cave. When exposed to air, the iron oxidizes or rusts and releases sulfur, which combines with water to make sulfuric acid. This weak acid welling up from below dissolved the limestone to create the passages and caverns we see today.
See our Coronado Cave page for more information about visiting and exploring the cave.