Ahh. Water saturating right through the thin concrete. Interesting theory. Looks a little dark for that, but maybe. I saw pictures of the other cooling towers, and it looks like they can exhibit very dark streaks if it's been raining.
I didn't know that they actually fill up any of the interior volume of the tower with the evaporator material and spray heads; I thought they were all arranged around the outside perimeter of the tower, at ground level. Thanks for the information.
Someone told me (long ago) that the concrete at the narrowest part of the tower is only two inches thick or so. It's easy to see vertical rebar in the damaged tower.
That was the tower I was in at the Gavin Plant in Southern Ohio. There are likely different designs for different plants, especially those built in significantly different time periods.