They can be quite interesting to explore, as part of a general hobby known as 'Urban Exploration'. Most cities have very clean storm drains, with the exception of those that use combined sewage overflow. You can find rooms and junctions, often linked by large tunnels and pipes.
You can usually enter at the outfall of the drain, or by popping a manhole. Manhole covers are usually very heavy, and are dangerous to open from below.

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