I am going to suggest another way to consider what is an open-air activity. Rather than say it is "outside" depending on whether there is some kind of connection with the outside world, whether it's a view of the sky, or whether you can see daylight, how about whether you are directly influenced by meteorological factors such as sun, rain or wind. We go "indoors" to escape the effects of the weather. We go "outdoors" to experience the direct effects of the weather. I said direct effects, like feeling the wind, the sun, or getting wet in the rain, or cold in the snow. Underground you are not partaking in open-air activity by that definition. Floods and underground streams are indirect effects by and large, insofar as they influence us as cavers. The cave floods because of the weather but the flood itself is not "weather", the rain is. So the only place you can say you are doing "open-air activity" when in a cave is when you can feel the wind, sun, or rain and that is not going to be very far into any cave bar a very few.