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. 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.
