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.
Some people live in huts or other types of human dwellings that don’t have entry doors. They have doorways.
When I was visiting the Philippines I saw homes on stilts that did not actually have doors, but I would certainly consider the inside of such a structure to be indoors. Pre-colonial architecture may not use a front door.
I recognize this isn’t a great argument, but it’s arguable I think in principle.
Pedantically speaking, I have never been in a door. I've opened doors and passed beyond the portal they were blocking, but I have never phased into or been sealed inside of a door. :p
If you go in the door you’re indoors but if you don’t you’re outdoor.
This could lead to a strange situation where you enter the cave through a doorless entrance and leave through an entrance with a door, making the outdoors indoors in the split second before you fade from existence.