Carlsbad Caverns, New Mexico

Created slowly over the millennia, Carlsbad Caverns' formations have been considered by many as the Eighth Wonder of the World.
Each movement of the earth, each drop of water, each puff of acidic air has left its mark on the caverns' formation. It began about 250 million years ago with the creation of a long reef in an inland sea. The sea eventually evaporated and left the reef buried under deposits of gypsum and salts.
Then, 20 to 30 million years ago, the Guadalupe Mountains were uplifted, fracturing the ancient reef. Groundwater seeping into these cracks combined with hydrogen sulfide gas from the oil and gas deposits below, creating sulfuric acid. This very strong acid carved out the large rooms and passageways that exist today.
Finally, about 500,000 years ago, after groundwater levels had dropped, the slow process of speleothem— stalactites, stalagmites, flowstone—formation began with a single drop of water. Drop by drop, and millions of years later we have what we see today!