The reason the subfloor had been able to absorb so much water over so much time was that the subfloor was built atop piles of white beach sand. Crane Beach sand? (Bob thought the sand had probably been gathered from a huge sand hill near our house, by an old cemetery.)
Opening the hatch also revealed problems with the floor sill. Ancient insects had eaten most of the floor joists and the wood sill was essentially gone. The room was resting on cinder blocks that someone, at some point in history, apparently recognizing that the room was sinking, decided to use to give the room some form of foundation.
(Cinder blocks and sand and old eaten beams.)
The cinder blocks must have been squeezed through the (now inaccessible) crawlspace, because the hatch hadn’t been opened in many, many years; probably not since the plumbing was first installed.