The term "Urban Explorer" encompasses pretty much everything from the amateur industrial archaeologist and photographer, to kids who like to break into abandoned mental hospitals because it's like soooooo Goth, man.
The issue of trespassing aside, some very interesting photo essays have come from urban explorers, including abandoned amusement parks, RR stations, mines, shipyards, and various tunnels.
Among the most haunting photoessays of industrial abandonment is by Shaun O'Boyle, who twice visited the Rossville Boatyard at Staten Island. O'Boyle's photos
are not to be missed.
Another of my favorite sites, very entertaining, tho less than scholarly, is the "Unquiet Tomb." Unfortunately it seems to be currently under reconstruction and not viewable.
Another good one is "Undercity."
As you might imagine from the name, the site mostly focuses on tunnels...and the obligatory abandoned mental asylum.
"Urbanadventure.org" offers some interesting views, including the Newark subway tunnel at what was Kresge's department store.
Is alot of this stuff illegal? Sure. Would we get to see these photos any other way? No.