Ideas for skill challenges in the Underdark?

So I was wondering if anyone out there could help me out with some more creative ideas for skill challenges in the underdark. So far I have come up with chasms, very tight claustrophobic caves to get through, hazardess fungi caverns, lack of oxygen, caves underwater, and more but right now I am drawing blanks (working nights, lack of sleep) Could anybody help me out by giving me some more ideas for skill challenges I can run in the underdark? I want them to be a big part of travelling far in the place, and I don't want them to feel stale everytime they go out on a big journey... Please, any ideas, even if they seem silly, would be appreciated. Thanks a ton.
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