According to the Guinness Book of World Records, Americas 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 cats bones have been preserved at New York Citys 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.