Hmmm, an "unexpected" sewer beneath one of the biggest, heaviest, fakoff manhole covers you can get?
Lol.
Is this a Time Team thread? If so you need to add a cheap Wilko trowel and 2" paintbrush to that set of tools on the IC lid.
My guess is that it's an old manhole repurposed to cover up an old septic tank or well. Either way you are now duty bound to get that cover off and report back with pictures! Don't try and get the cover bit up, just dig around it and slide it off the bricks with a crowbar.
Edit: Also that brickwork looks far too shoddy to be part of a proper manhole, which would seem to support my theory above.
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