H/T to Terry for sending!!!
Exploring the old railroad tunnels atop Donner Summit in the Sierra Nevada mountains, which were blasted and picked by hand by mostly Chinese rail workers in the 1860s. When the railroad was rerouted in 1993, these tunnels were abandoned -- and now anyone can just walk right in and check them out. It blows my mind that something with such historical significance is just sitting there unsupervised -- but it's TOTALLY FASCINATING to explore!