According to the Guinness Book of World Records, America's largest underground lake is tucked away underneath Tennessee. First, you embark on a tour of giant caves 140 feet below ground, and then hop on a glass bottom boat to float across "The Lost Sea". Rare collections of cave formations and 20,000 year-old jaguar tracks await you. (The ancient cat's bones have been preserved at New York City's Museum of Natural History.) Another plus; the temperature is always 58 degrees, so whether you want to escape from a sweltering summer day, or a chilly winter, this is an ideal pit stop.