A very long time ago, a shallow sea covered the area of what is now Carlsbad Cavern. Plants and animals lived and died in the sea. Their shells and skeletons piled on top of each other, making a reef. Over time, many layers piled up, squashing the shells and making the layers hard, compact and thick.

Then the sea dried up, causing the reef to be exposed to the air. Movements in the earth's crust pushed the reef upwards, forming a limestone mountain. Trees and other plants grew on the mountain, covering the old reef and causing cracks to develop in the limestone. Rainwater sank into the soil and went down through the plants' roots and finally down through the cracks in the limestone. On its way through the atmosphere and the soil, the water absorbed carbon dioxide. A weak acid was chemically formed when the water mixed with the carbon dioxide. The resulting carbonic acid dissolved the calcite in the limestone.

At some point, large rocks in the cave ceiling fell. This opened up chambers, like the Cavern's Big Room -- 25 stories high and a third of a mile wide.

As water seeped and dripped its way into the Cavern, beautiful formations decorated the cave.