Whilst we were digging drain runs....

...we found a hole, the digger driver, Keith, had to remove a large stone along the pipe route and found a hole full of water. He used a 4m scaffold pole which was lying around the site to see how deep it was, the pole went in nearly the whole length and hit the bottom, with the water over 3.5m deep.
So having no idea what it was we had to arrange to get the hole pumped clear of the water so we could see what was down there and how safe it was.

See the hole in the foreground, it was covered by a large stone

Measuring the hole, note full of water

Camera view as no way to get head in hole! Note the roof is arched and made of stone

Scaffold pole being used as a measure...

...this is how deep it is, it swallowed a whole length of scaffold pole!

Once it was pumped out we could see a little deeper and could make a photographic record. The sides seem to have been made of rock, fairly sheer and no drain holes, so it could have been a well. It is near to the house so not sure if it would have been foul drain, more likely a well?

Looking down from above, see two sides very smooth, other sides are chiseled, width about 1.2-1.4m

Side view, quite smooth considering nearly 4 m deep and presumable cut by hand

Arched stone roof, only about 300mm below ground level

another view from above, all taken by pointing camera into hole

view of hole from above ground level, showing arch stones.

After the photographic record and a video was recorded, for safety the well was filled with 20-40mm stone, both to support the well, the roof and in case anyone came across it and fell in, especially dangerous if filled with ground water, due to smooth sides and small space. 
It has been recorded on the National Trusts record system for that property.