It’s not difficult to pair a visit to Mammoth with others

Civilization has come a long way since some prehistoric ancestors sheltered in caves, but these chilly, damp holes in the earth continue to fascinate.

Mammoth Cave, in southwestern Kentucky, was one of America’s first international tourist attractions, opening to the public in 1816. Niagara Falls was the only attraction to outrank it.

Kentucky has more than 3,000 known caves and about seven show caves. Most caves are under private property and thus have private ownership. In fact, Mammoth Cave, the longest at more than 400 mapped miles, is only partly under national park land. The rest belongs to landowners above ground.

In three days visiting southwestern Kentucky, we visited three caves and had distinctively different experiences because of cave composition and crowd size. Small crowds are better for both listening and asking questions.

Our first experience was Hidden River Cave, in the 2,200-population Horse Cave. It is owned by the city and operated by the nonprofit American Cave Conservation Association. It starts with a 173-step descent down a stone stairway into damp, earthy-smelling cave air. In a well-lit hike through spacious passages, visitors find themselves 150 feet under city streets, where the temperature is in the 50s year-round.

At the cave opening, the tour guide paused to talk about history. American Indians used the river cave for its clean water and other resources. In the late 1800s, a private European owner began selling freshwater from the river. Then, in 1916, the stone steps were built, the first electric lighting was installed and it opened to the public.

After showing off rusty remnants of the early electricity-generating equipment, the guide got sadly serious about pollution that shut down the cave in the 1940s. Lacking a sewage processing system, local land, sewers and industry dumped into the lowest point, the river. It wasn’t until the 1990s, with pollutants removed from the cave and a regional sewer facility in place that tourists could return.

The makeup of this river-cave is such that it lacks stalactites, stalagmites and other structural