In article ,
Andrew May writes:
Full marks for everyone who identified it as a septic tank. Based on the
information provided here I excavated further to the left of the first
chamber and came across what can only be described as a standard
inspection chamber. Straight through with a pipe from the house and
exiting into the first tank. I have added photos for the benefit of
anyone not by now terminally bored by this thread.

I couldn't see any new pictures, although the page was just updated.

The one oddity is that the pipe that I thought was the inlet to the
first chamber in fact comes from above the water level in the inspection
chamber and the inlet to this is blanked off with a clay disk sealed
with pitch. This all seems to be a single piece of ceramic so I assume
it is intentional. Is it some sort of safety valve in the case of
exploding gasses?

You might be describing an interceptor trap, but difficult to tell
without a picture.

Thanks for all your help. Now I know what it is I can start dismantling
it and planting the garden.

A pity there' nothing useful to use it for.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]