Extreme caving: a tube passage
Unknown location: sometimes you need to put a manmade tube into a cave passage to avoid continuous build-up of debris in this section, but also to avoid cavers getting buried under debris in this section, typically in the very early regions of the cage as you can't transport such tubes deep into a cave at all.
These tubes are obviously just sufficiently wide for a skilled and fearless person to get through. This (very short) clips gives you an idea what it feels like diving in there.
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