Carlsbad Caverns National Park … in the Guadalupe Mountains of southeastern New Mexico. The primary attraction of the park is the show cave, Carlsbad Cavern. …

Carlsbad Cavern includes a large cave chamber, (The Big Room), a natural limestone chamber almost 4,000 feet (1,220 m) long, 625 feet (191 m) wide, and 255 feet (78 m) high at the highest point. It is the fifth largest chamber in North America and the twenty-eighth largest in the world. …

They do a good job at Carlsbad. But the most interesting thing I learned was the existence of the much larger and more impressive Lechuguilla Cave nearby.

… with 138.3 miles (222.6 km) the seventh-longest explored cave in the world …

They are not likely to let tourists into this one in my lifetime.