4. Sequoia Tree Tunnels
Sequoia National Park, California
The iconic Wawona Tree tunnel may not be standing anymore -- it finally collapsed during an especially rough winter in 1969 after, uh, 2,100 years -- but you can still squeeze through the fallen Tunnel Log in Sequoia National Park in central California. An 8-foot-tall tunnel bored through the body of a 2000-year-old tree may not be one of the most daring feats of engineering on our list, but it gets bonus points for making you grapple with your own mortality.