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 say outdoors. Indoors has a vibe of being human made to me, and a low likelihood of encountering wildlife. So I guess walking around a mine is indoor cave exploration
Many caves are modified to accommodate their visitors. This could be a small as leaving behind oxygen tanks or equipment, or as advanced as installing electrical wiring and ventilation. This was the case of several caves I visited as a tourist with no real exploration skills.