7.  Old Sacramento Underground, California

The city of Sacramento was blasted by 45 days of Pacific storms in 1861. All the rivers surged through the city and swept thousands of people to their drowning deaths. Rather than run, the residents had a plan  

They would raise their homes and streets by nine feet. Today you can tour the abandoned alleyways, basements, and underground streets thanks to the Sacramento History Museum. You can even catch a live virtual tour on their website.

8.  The Inner Space Caverns, Texas

The exotic Inner Space Caverns were accidentally discovered by the Texas Highway Department in 1963. They were drilling through the rock there to determine if the rock could actually support an overpass. The drill dropped through the roof of what is now named the Discover Cave. It was shortly after that they discovered they were standing above a large subterranean network of caverns complete with still pools, sharp stalagmites, and coral from what was once actually the Pacific Ocean floor.