This hobby is shared by people across the world who are curious about the built environment. Like other forms of tourism, 'UE' involves finding and learning about sights, going out to actually see (and usually photograph) them, and talking about it afterward. In this particular instance, the sights are things like abandoned buildings, sewers, tunnels, rooftops, and other areas that are generally out of sight. It's pretty fun, no matter how you do it or what you call it, especially if you bring a friend or two. As with any other semi-dangerous hobby, being aware of the risks and acting responsibly is vital.
Much of what I'd say to explain Urban Exploration has already been well said by someone else. If you'd like to learn more an excellent starting point is Infiltration.org There is a ton of amusing and well-written information on the website.
There are many books published about 'urban exploration', go check out your local library. I would highly recommend "Corporate Wasteland", by Steven High and David Lewis, or "Invisible Frontier" by LB Deyo and David Leibowitz.
I could go on at length about the underground systems of Edmonton or Vancouver, but I'll just sum a few basic facts. Drains (slang for storm sewers) are pipes that convey rainwater runoff from roads and parking lots to the rivers and ocean, often without treatment; generally referring to a trunk pipe big enough to at least crouch through, although many are quite large. There are also sanitary and combined sewers, which I prefer not to enter because they contain raw sewage and deadly gases. Edmonton has many storm sewer pipes and structures located across the city; many of which are quite large, built to drain large amounts of water. These sewer pipes are not meant for human occupancy and there are many dangers present; sewers should not be entered by the unprepared (and never when it rains!).
I've done a lot of thinking about what 'urban exploration' means to me. The world has changed a lot since I got into this hobby decades ago, and I've changed too. I always enjoy learning about and visiting the more obscure, secret, and historic places in my area; even if I don't do much of it anymore. Stay curious, stay active, cherish the moments you share with friends.
This website was a travelogue, where I shared accounts of my explorations solo and with friends. This website, DrainsofmyCity, has been online since late 2001, steadily grew to a peak around 2006, and slowly declined after 2008. Please don't use any or the words or photographs without permission. Or maybe this has all been a work of fiction?
Please feel free to email me- I like hearing from other explorers, history buffs, or visitors who enjoy the website.
E-mail: reduxzero -at- drainsofmycity -dot- com
If you're looking for directions to a location or information on how to get in somewhere, don't bother asking. Finding new sights is part of the fun.