Archaeologists at Bath Abbey have unearthed the remains of a Norman cathedral, thought to be the first ever built on the site.
The foundations, which stand 3m to 4m high (9ft to 13ft), have been buried for several hundred years.
Archaeologists believe that they may have also found what is left of a medieval abbot's lodgings nearby.
A series of digs are taking place which aim to discover the best way to develop the venue in the future.
Bath Abbey is an active Christian church, located in the heart of the city, and was founded in 1499.
"It's a rare opportunity to dig in the centre of such an historic city," said Mark Collard, from Cotswold Archaeology.
"We do an awful lot of work which is very mundane but this really is a privilege here."
It is hoped that the dig will also help the abbey to expand its facilities.
"Firstly we're looking at the possibility of putting new rooms underground," said Charles Curnock from Bath Abbey.
"We need new facilities here: toilets, storerooms, choir rooms and so on.
"Inside the building it's quite different. There we are looking to provide a stable floor because the floor is collapsing."
The archaeological digs are expected to continue until Easter.