Is cave exploration an indoor or an outdoor activity?
You’re indoors in the sense that you’re protected from the weather and the elements, and the cave could even have some kind of covering or entrance area that could be considered a door or doorway. People have built homes in caves.
I agree with the door argument but would also add my two cents... If it's a cave dwelling it would be inside but you also wouldn't be cave exploring. You'd be in a 'civilized' area