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Is cave exploration an indoor or an outdoor activity?
(infosec.pub)
A "Showerthought" is a simple term used to describe the thoughts that pop into your head while you're doing everyday things like taking a shower, driving, or just daydreaming. The best ones are thoughts that many people can relate to and they find something funny or interesting in regular stuff.
I would consider a cave to be "outdoors" unless maybe somethings been done to it to make it into a dwelling. Since there is no door it's at least somewhat open to the elements, you would have to contend with various wildlife that may live there, the floor would be either dirt or bare rock, these are all things one would have to deal with while doing "outdoor activities"