Seneca Caverns, Ohio
These subterranean caverns were discovered accidentally in 1879 by two boys at play who fell through a hole. In 1931, explorations continued where tunnels led further into the depths. Now you can explore these depths too aided by a trained guide. One of the highlights of the cave is the underground stream of crystal clear water. You can also mine for gems and fossils - although the baubles are not native to the cave.