This is a hobby shared, to various degrees, by people all over the planet. The common thread seems to be an innate curiosity, which manifests itself in the drive to discover our environment, which happens to be urban. By going places others normally do not go, and seeing things other rarely see, we are able to learn about and gain a deeper knowledge of the world that we've build around ourselves.
Nearly everything I would 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. From the well-written No Disclaimer (go read it!) to the informative Infilspeak Dictionary, there is a ton of information on this website. Also, the Worldwide Links page is probably the most complete directory of Urban Exploration and related websites on the internet.
I could go on at length about Edmonton's underground world, but instead I will boil down a few basic facts. Drains (slang for) are storm sewers which convey water from the roadways to the creeks and river; the sanitary and combined sewer systems are different pipes that I do not explore because they are filled with sewage, unpleasant, and very dangerous. Edmonton has many storm sewer pipes and structures, located all across the city; many of these are quite large, built to drain large amounts of water. The storm sewer system is not meant for human passage and there are many dangers present; exploring drains should not be attempted by the unprepared (and never when it rains!). For more general (non-exploring) information and facts about the sewers, the City of Edmonton's Drainage Services website is helpful; and I've also located a useful Glossary of drainage terms.
As an Urban Explorer, my main focus is to visit and learn about the more obscure, secret, and historic places in my area. I do this partly for the thrill that accompanies discovery, partly to learn more about the city I live in, and also for the sense of accomplishment that follows succesful exploration. And, on top of all of that, Urban Exploration is fun. Whether I'm exploring alone or with others, Urban Exploration helps me feel alive; heart pounding, eyes flashing, mind racing, on the smooth flow of a good outing. I do explore more than just drains- abandoned buildings, tunnels, bridges, bunkers- and although the missions aren't uploaded to this website, there are photos. I also enjoy traveling to other cities and provinces to explore. Here's how to contact me.
As a byproduct of exploration I've taken up photograpy; which I enjoy because it allows me to creatively record the places I've been and to illustrate them visually to others. This website provides a venue for me to write accounts of my explorations, to detail my actions and express thoughts and feelings. This website, DrainsofmyCity, has been online since late 2001, steadily growing and changing; this is version 6. I personally design, write, photograph, create, code, and publish everything that goes online (exceptions are noted); as such all content is copyright and may not be used without permission. The website uses valid HTML4.0 and CSS.
As mentioned above, the Worldwide Links section from Infiltration.org is a comprehensive, excellent resource.
For the drainers, a few must-visit websites:
International Urban Glow, which has Siologen's stunning photography from sewers and other places around the world
Vanishing Point is Kowalski's excellent website on draining in Ontario
Sub-Urban, UK explorers JonDoe and Stoopmaster explore some of the most amazing drains I've ever seen.
Some of my Canadian favourites include:
Urban Exploration Alberta, which is the Calgary community
Wraiths are a stealthy group from Vancouver
Urban Exploration Montreal have frequently amazed me
Corrosion is Mr. Sable's home on the web
Urban Exploration Resource is a bee hive of discussions