Colossal Cave is a large dry limestone cavern about 25 miles southeast of Tucson, in the foothills above Vail. The Civilian Conservation Corps built the stonework stairs and walkways in the 1930s; the standard tour takes about an hour and a half and descends a few hundred feet underground.
There is no live cave decoration like at Kartchner, but the rooms are vast, dramatic, and a steady cool 70°F — useful in summer. The surrounding park has trails, a butterfly garden, and an old ranch with longhorn cattle. A worthwhile half-day from Tucson.