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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. A showerthought should offer a unique perspective on an ordinary part of life.
Caves are outdoors because a bunker is indoors and the cave around it is outdoors.
Just for the sake of argument and fun:
Aren’t we all outdoors because we are living in buildings that are themselves outside?