Billt4SF wroteNot exactly on point, but I read recently that rats have spines that an bend both ways, hence it is easy for them to come up a sewer line and through the business end of a toilet.
- Bill
That is close Bill. This guy is definitely a water lizard, and I today found his access.
A couple years ago had a new septic tank installed and it was connected to the previous one which was not blocked. As a result a rather a somewhat larger vent was left unplugged and believe it provided the access.
The rascal is still there and must have an iron constitution (no explanation), but if anyone has a thoughtful solution it would be welcome.