And I wouldn't think it would be a well. My sister has an old well on her property, and it was mentioned in deeds - indeed, the neighbours had an easement for access to it for water. Not a good idea, since when the mains were attached, it was converted into a cess pit. Also, a well would need a way for water to enter, and would not be so wide. It sounds quite modern, of a time when a borehole would be more likely.

Does the soil look as though it was all put down there at the same time? And does there appear to be anywhere in the garden where the waste that is not in the chamber could be? (Irrelevant to what it is, perhaps, but contributing to the overall picture.) Does the state of the top look as though it was used at all - is it worn? Can't be someone's nuclear bunker - too small!

Penny