Posted by blackhawk Make sure there's no possibility of storms for the duration you'll be there. Be wary of snow melt off in the winter. Smaller diameter/capacity drains are more risky... a freak water main break could trash you on a bone dry day. When in doubt, bug the fuck out.
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I've heard of underground reservoirs or caverns in larger drains that gradually fill, and serve to slow the flow of water through a drain system by holding large amounts of it. When these spaces reach their capacity, they then vent the water through overflow drains, much like the overflow drain in a bathroom sink when the main drain is plugged.
I've been told that this can happen even on a perfectly dry day, and can result in underground flash floods with no precipitation or water main bursts. Now, I've never seen one of these myself, but they are yet another thing to be concerned about underground.