You may be lucky----that 'wood' under the toilet? Is it dissolving into saw dust?
Looks like the floor was laid wrong many years ago---
Remove that layer and let's see what's under it----could be 1x6 solid wood or might be another layer of floor covering---
General instructions---1)floor joist (strong enough to support natural stone) more on that later.
2) Subfloor--in 1950s and before--1x6 solid wood--Plywood in newer structures
3) additional plywood to isolate the solid wood from the tile.(solid wood moves to much and can cause tile failure.
4)tile backer--usually 1/4" cement board--set into thinset like a giant tile and nailed or screwed--
sometimes--a membrane like Ditra,by Schluter
5) tile or stone set into Modified thinset (if a membrane is used see what the manufacturer recommends)
Looks like the floor was laid wrong many years ago---
Remove that layer and let's see what's under it----could be 1x6 solid wood or might be another layer of floor covering---
General instructions---1)floor joist (strong enough to support natural stone) more on that later.
2) Subfloor--in 1950s and before--1x6 solid wood--Plywood in newer structures
3) additional plywood to isolate the solid wood from the tile.(solid wood moves to much and can cause tile failure.
4)tile backer--usually 1/4" cement board--set into thinset like a giant tile and nailed or screwed--
sometimes--a membrane like Ditra,by Schluter
5) tile or stone set into Modified thinset (if a membrane is used see what the manufacturer recommends)