Jewel Cave was declared a national monument in 1908, eight years after it was discovered by prospectors. Since then, spelunkers have mapped more than 120mi of passages, a mere 2 percent of what is estimated to exist. In the world, only the Mammoth Cave system in Kentucky (with more than 350mi mapped) is known to be longer. The 80min Scenic Tour starts with a 234ft elevator descent. Visitors then follow a concrete path and aluminum stairways to 370ft below the surface, where it is a constant, cool 49ºF. Massive chambers glitter with the calcite formations-elongated crystals of dogtooth spar and nailhead spor-for which the cave is named. On the 1hr 45min Candlelight Tour, visitors carry candle lanterns while following a half-mile of early explorers' paths. Before going on the very strenuous 4hr Spelunking Tour, visitors must first show they can squeeze through an 8.5in-high, 24in-wide block on the patio.