9. Great Basin National Park, Nevada

The "pearly gates" are one of the thousands of calcite formations that visitors can see in the marble Lehman Caves inside this park, which is about equidistant from Las Vegas and Salt Lake City. Guided tours inside these caves reveal beautiful stalactites, stalagmites, helictites, flowstone, popcorn and rare shield formations year-round. Visitors in 2009 (through August): 60,248 More info: Pictured: The "pearly gates" cave formation Brandi E. Roberts / National Park Service