It all started with pulling off a loose tile...

Wellp, I was planning to mortar up the damaged drywall to reinforce it and then stick on some new tiles using the mortar and some fresh grout. But the drywall crumbled when I touched it... revealing insulation that had decomposed into dirt. Behind the insulation were house beams disintigrated by dryrot. It is a horrid mess and I have no idea how far it extends or if the mess includes the upstairs bathroom as well. The dryrot had progressed to the point where if those beams were supposed to serve a purpose (and I suspect they were), that purpose is no longer being served. Furthermore, the dryrot looks like it extends under the tub. One corner of the tub was rusted away (which didn't show until I pulled a tile off), and moisture was escaping below. From what it looks like (without taking down the whole wall to be sure), several house beams in two walls and the floor have to be replaced and/or reinforced, after which all the drywall, tiles, and fixtures have to be replaced.