>>2362322A long dark tube with some water, probably some dirt and spiders. Bats sometimes. Lots of graffiti.
>>2362336A spare flashlight
Fresh socks for when you get out
Consider a hard hat (you will bump your head)
>>2362351Your main concern in that regard is oxygen displacement and harmful gasses. Although it depends on what kind of fancy future resp protection you have, but it probably won't spontaneously produce oxygen for you to breathe and block all of the harmful gasses you want to avoid. I generally recommend not wearing anything aside from particulate protection (for asbestos, if you expect it), as you don't want to walk around anywhere you need a mask. Instead, buy a detector and gtfo if you smell anything weird or it starts beeping. Most filters will also the smelly vapors associated with harmful conditions, so people don't like to wear them for that reason as well.
That being said, gas is only a significant concern if you're going into a large system with low use, it won't be a concern if you're walking through a 10' highway culvert.
