This was a double post so I'll edit it & add a bit more.
Obvious dangers in drains.
Losing your head popping manholes in a road.
If you don't know the drain, don't pop manholes unless it's a rectangular concrete cover as I've never seen one in the road.
Flood.
When it rains, no drains.
Falling down a ladder.
Always take a friend.
Gasses (I've explored drains for 40 years & only came across gas twice, but that's in Australia).
I'd head back the way I came quick smart.
Tingling fingers is not a good sign.
I've heard stories of people jumping into confined spaces to rescue someone that is overcome only for the gas to overpower them as well. That's when you need Rescue Me 1060
If I was caught in a bad flood I would climb up the nearest ladder. If it's not a Summer storm then chances are you'll be fine, but still, when it rains no drains.
Cheers