Have you ever been high up a mountain, and found yourself short of breath? That was because the air gets thinner the higher we climb.
At what we call ‘sea level’, we don’t really notice the air. In fact, except when it gets foggy, or starts filling up with water (rain), it’s usually invisible. We can’t see it, so we take it for granted.
For a moment, consider the fact that it’s actually physically there around us, all the time, in exactly the same way that the sea is around fishes and crabs. We live in a sea of atmosphere, and we would die as quickly as a fish if we were taken out of it.
We are undersea creatures, and we can’t even swim in our own sea (though birds can, as fish can in their own sea). We are more like crabs than fish, most of the time scuttling over the very bottom of our sea. Yes, we have invented aeroplanes, to allow us to mimic fish. It makes the fundamental fact all the more obvious.
Just as we are creatures of outer space, we are undersea creatures.