Oh my, look at the floor! There must be something about gymnasium flooring that makes it do that rapidly. We have soem abandoned schools and community centers in my town that have only been empty since the nineties, and the gym floors have all done that, just collapsed where they get stepped on. Yet the classroom floors are fine.
I especially like the wave sort of texture the floor has formed as the bearers decayed, it's remnant of the patterns water rents onto sand, or the similar effect wind does to land, only visible from a great height. These effects bring it back to nature, like the slow decaying process is is the same slow process of creation.