Rivers, streams, and other such beautiful natural waterways are all well and good, except when they get in the way of good old-fashioned human expansion. The sad fact is that most streams that stray too close to a modern city are usually diverted, piped away and paved over with no quarter given. The few signs of this left top-side are generally taken for granted: gutterboxes, grates, manholes, pipe mouths in ditches and outpours in holding basins. A few feet beneath these mundane features, however, things are far more interesting.

Storm drains are tunnels run under a city designed to divert rivers to designated areas, in order to reclaim the land they take up and provide a network to drain rainwater that collects on the streets. These are a completely different beast than the oft-maligned "sanitary sewers", which are the exclusive carriers of raw sewage, foul water and other such euphemisms for "crap". Those we stay away from. On the other hand, storm drains, which normally carry only rainwater and street runoff, are considered fair game for exploration.
Why? Because they're there! Storm drains, from the unremarkable round concrete pipes of our home city, to the ridiculously oversized cavernous wonders in Australia, are somewhere that few people actively think about and even fewer actually venture into. This is a shame, because within these man-made caves one can find all sorts of sights that you'd never expect to have been under your feet all this time. Mineral deposits, stalactites, ice, graffiti from past explorers, huge chambers, ladders, balconies, pits, slides, spiders, and all manner of other features can be found in the storm drains of your city. In a drain one finds experiences completely new to someone who's spent their life topside -- venture a few hundred feet into a drain, switch your light off and look around you. It's pitch black, not a photon to be found. There is almost nowhere topside in a modern city that is ever as dark as within the depths of a storm drain. Listen to the sounds -- the running water, the echoing footsteps. Give a woop down a tunnel and you'll hear it come back at you a full second later. The sight of two small beams of light, the only light to be had, shining through the humid air from a manhole several feet above you is unforgettable.

Everything is different underground -- things like the thump-ump of cars hitting manholes over your head, or the occasional surreal conversation with someone who hears you and your partners conversing beneath a grate and yells into it, not quite believing that he's shouting into the ground. The looks passers-by give when a manhole on the sidewalk suddenly lifts itself out of its collar and a troupe of drainers emerge, nod, slip the lid back into its place and walk away is priceless.
There are a certain number of things one should be sure to take on a draining expedition. First, and most obvious, is a light source. Photons are a rare and precious resource in the tunnels, so if you're venturing in you've got to be sure that you've got a reliable light. An ideal flashlight is waterproof and won't sink, as well as having a long lifespan -- both bulb and batteries. Drains are a tough environment on lights and your average NoName Brand Dollar Store Flashlight (TM) will work for one trip (maybe) but it won't last much longer than that. Corrosion, bumps and splashes are just a few of the nasty things your flashlight will be exposed to so don't be cheap. Currently UEC is having a blast with a flashlight recently put out by Radio Shack -- a tiny, portable clip-on light with two high-intensity white LED's that runs from three AAA batteries. It's highly recommended if you can find it -- the light output is more than sufficient for a drain and the battery life can be in excess of eighty hours. The LED's themselves will almost certainly last longer than the rest of the light does, so bulb-changing is a non-issue. In fact, you should always use LED lights when they're available. Screw incandescents -- they're glorified space heaters. LED lights are more reliable, use less energy and produce less heat. They're more shock-resistant and last longer. Use them. If Mag-Lite would produce a good LED flashlight with the usual fantastic body construction, I'd sell all my other light sources.
At any rate, while draining alone is fun and an interesting experience in and of itself, it is highly advisable to always bring a friend or two. If you're wounded or incapacitated while by yourself in a drain, you're basically screwed. Having a partner on an excursion means there's someone to go for help or get you out. Either way, you should generally tell someone where you're going before you go into a drain, in case you don't come back. Er.

Other equipment that's good to have in a drain includes rubber boots, crowbars, crayons (if you're the tagging type), and rope. Improvise -- bring whatever you think you'll need and make up your own list for excursions that strikes a balance between weight and utility.
Draining isn't for everyone, and could almost be considered one of the fringe areas of urban exploration. Some explorers swear by drains and rarely do any other, top-side targets, whereas other active explorers avoid drains in general unless they offer something truly fantastic. Some people are just drawn to it more than others. It's the "getting wet" part that puts most people off, I believe.
Draining is dangerous, exhausting and requires you to be alert and responsible, but it also offers many fascinating sights and experiences. Knowing how to get from place to place in your city from underground is a unique and nifty thing.
There are many reasons to go into storm drains, but just as many reasons not to. Draining is a dangerous act -- every time you enter a storm drain, you're taking your life into your hands. Pollution, bacteria breeding in stagnant water or in sanitary overflow from a recent storm, heavy water current, steep falls, unsafe ladders, sharp objects washed in off the street, and other hazards can be present in varying amounts. If you're in an urban area, there's a good chance that local industries use them as convenient venues for illegal dumping of materials you'd probably be happier not getting mixed up with.
There are several designs of gross pollutant traps, designed to remove large sediments from the water before it reaches the lake, and they vary from holes in the floor to elabourate designs that look somewhat like washing machines from the inside. Among these are the dreaded Stormceptor, a complex device which spells certain death for any drainer unlucky to find himself washed into it.
Draining is illegal in most places and in North America is a civic trespassing charge -- a ticketable offense of up to about sixty dollars. However, from country to country the laws vary wildly so you'd do well to check locally.
While there are many good documents on draining which go into full detail regarding the dangers involved, other things you should be aware of include the chance of poisonous or explosive gases, as well as oxygen deprivation.

Don't ever drain in the rain ("when it rains, no drains", as the Cave Clan adage goes), even if you think you know a drain's watertable intimately. You can still be surprised by a sudden rush in the current and be knocked off your feet, soaked and dragged along several feet at best -- at worst you can be completely sucked underwater and thrown at a high speed over a ten-foot waterfall onto concrete. Draining is dangerous, and anyone who wants to try it should be sure that they have the maturity to take the responsibility for their safety totally into their own hands. There's nobody trying to keep you alive down there -- like most places in urban exploration, the drains are a truly wild environment. Take every precaution and be sensible.