Followup
If you look beyond the US, there are even more abandoned places.
The obvious example, not on that list, is Chernobyl, which can apparently be traveled in parts.
I think Andrew has posted about Gunkanjima before, an abandoned city in Japan.
But what fascinates me is the second one on the list, Varosha, Cyprus. A perfectly-good city, a beachfront paradise, even, was abandoned (well, the residents were forced out, apparently). What’s odd is that, vacant for thirty years, the military aggressively patrols the area, not just with myriad guards outside but, apparently, guards inside in case anyone jumps the fence. Even photographing the city is forbidden. (Which, of course, adds to the intrigue and, subsequently, the desire to photograph!)
How interesting it would be to explore! (When you’re done checking out the Cyprus photos, check out the rest of the site, which is also fascinating!