...oh, that is very familiar, isn't it? I didn't even think of it until you brought it up. It's not on the tip of my tongue, but it's close; I'll be struggling with this all day.

I'm sure it involved the Moon, rather than Mars. I'm positive on that. Trapped pockets of atmosphere, just on and under the surface, most under, I think a lost girl and boy discover some sort of grotto of it, getting lost, perhaps while running low on air?

Every time I grasp at it, it slips away. It... feels in my head a bit less pulpish than it might sound from what I think I'm recalling. If I come up with it, I'll drop another note. I wonder if the reference librarians over at Suzzallo might know.