Wiltshire archeologists say it is likely to be a 4,500-year-old megalithic tomb.
They have since surveyed the site, and believe there may be several more of them in the same field.
The burial chamber was discovered back in the summer when the Sarsen capstone collapsed and a small sink hole appeared in an arable field.
“For I ages I thought it was just another badger set but when I examined it more closely you can see that it is lined with stones and there is some kind of chamber there,” said farm owner James Cameron.
“It isn’t very big, probably a few metres square and dates from the Bronze Age.”