Explore 8 Midwest National Monuments

Walking inside the world's second-largest cave—160 miles mapped so far—is like walking into a geode: rough on the outside, lined with white crystals and full of sparkly surprises. Nailhead calcite gives the Custer, South Dakota, cave its nubby walls that can look like coral; dogtooth spar provides the pockets of pretty crystals. Feel the cut-glass texture on a "sacrificial rock" at the beginning of one of four tours (discovery, scenic, historic lantern and wild caving). Aboveground, three trails illustrate how surface and subsurface interact (605/673-8300; nps.gov/jeca).

Jewel Cave National Monument