From water heater explosion?
I am now wondering if the hole is related to an exploded water heater tank. I pulled the tank out tonight because it had a huge leak. I am glad I did because it turned out to be such a mess (pictures are in another thread, "water heater woes.") Here are some more pics, of the nice clean white OSB shelf the water heater was sitting on, and the mess below.
If the exploding tank was responsible for the hole in the floor and wheel well, or if the same thing that made the hole also destroyed the water heater tank, somebody did a cover-up! The nice clean white shelf was in between the tank and the hole. It wouldn't look so nice if the two were related.
I guess I will fix this by smoothing the hole in the OSB, cutting a plywood plug to match, and glassing it in top and bottom. Same on the wheel well portion, without the plywood plug.