On the other side of I-10 from Colossal Cave, one of Arizona’s most long-awaited “openings” was that of Kartchner Caverns State Park (520/586-4100), a massive limestone cavern—over 2.5 miles long—that is rated by experts as one of the 10 most beautiful in the world. It was discovered by a pair of avid cavers, Randy Tufts and Gary Tenen, back in 1974. It then took 25 years of negotiation and more than $28 million worth of careful construction of tunnels and facilities before the cave was opened to the public. Advance reservations for one of the guided tours (around $25) are all but required if you want to enter the cave to see the 150-foot-square Throne Room, with its 60-foot ceiling, or the larger but less lofty Rotunda Room, or any of the other phantasmagorical sights. The caverns, which are kept at near 100 percent humidity by a set of airlocks at the entrance, are remarkable for their diverse and delicate formations, including over 30 different types of stalactites, stalagmites, columns, draperies, shields, and helictites, not to mention the longest “soda straw” in the United States—a thin tube of limestone over 21 feet long but only a quarter-inch in diameter.

Kartchner Caverns State Park is just west of Benson, nine miles south of I-10 exit 302, along Hwy-90. If you don’t manage to join a tour, it’s still worth stopping at the 23,000-square-foot visitors center (520/586-4100, daily, $7 per vehicle) to see the movie describing the cave and its discovery. There are also some full-scale replicas of the cave’s features, along with an above-ground hiking trail through native hummingbird habitat, a 62-site campground with full hookups, and four camping cabins.