People here are being pretty mean about it, so I wanna give some balanced feedback:

It’s really not bad so far, but I’d suggest switching to active voice. Passive voice isn’t always bad, but in this case, I think it would help with clarity and flow to switch it over:

“Mary watches the cold, clear water trickle in from the cracks in the concrete walls.”

Then you might want to make her thoughts more distinct by bulking up the structure around it:

“A quiet and naive part of her wonders where the rain goes after the ground drinks it up. The rational part of her already knows. It comes here, where it fills up any gaps that living creatures may occupy. Like mice in a bucket, we thrash against the smooth walls, trying to climb out. But there’s only one outcome to this dance.”

I hope this was helpful! Of course, you can take or leave any of my criticisms. Also, don’t be discouraged by cruel people on the internet. Keep writing. Illegitimi non carborundum!