10 Amazing Underground Walks

Photo: Stephen Alvarez [National Geographic]

If you're looking for something to do on your next vacation that's a little different than the usual sightseeing fare, National Geographic has 10 jaw-dropping suggestions that all take place underground. That's part of the Mammoth Cave National Park in Kentucky in the picture above. Other suggestions include the Underground City of Montreal, the Cu Chi Tunnels of Vietnam and the Berlin Nuclear Bunker in Germany.

http://travel.nationalgeographic.com/travel/top-10/cave-tunnel-walks/