An 18th-Century cold bath, which may be the only one of its kind in an elite social meeting place known as an assembly room, has been uncovered.
The cold bath was filled in with rubble and covered after bombing during World War Two.
Its location in the Bath Assembly Rooms suggests it may have been provided for those wanting an "exclusive experience".
The National Trust's Tatjana LeBoff said the find was "highly unusual".
She added: "There are many elements of this discovery that are still a mystery.
"It is a rare, if not unique, surviving example, and possibly it was the only one ever built in an assembly room.
"We are still researching records, letters, diaries and other documents to see what more we can find out that will help us piece it all together."
The National Trust, which now looks after the Assembly Rooms, said ongoing research will draw on this "rare archaeological evidence" to piece together more about the bath and how it was used.
The excavation work, carried out by Wessex Archaeology, involved removing tons of rubble in the basement to reveal the steps down into the cold bath and a niche which would have held a statue or sculpture.
The rubble is likely to be from when the Assembly Rooms were bombed in World War Two.
Archaeologist Bruce Eaton said: "Although historical records indicated that there was a cold bath buried beneath the Bath Assembly Rooms, we had no idea what preservation of the bath would be like.
"The building suffered damage at the hands of the Luftwaffe and the rooms were remodelled in the late 20th century."
Bath is well-known for its hot spring waters, but in the 1700s, medical professionals were also recommended cold bathing, the National Trust said.
This would involve plunging into cold water for a short time, then quickly warming up afterwards.
It saw a trend in installing cold baths in private houses and estates, but also in public facilities.
The location of the bath inside the building though indicates it was likely more exclusive, the trust added.
The Assembly Rooms were built between 1769-1771 by John Wood, the younger, who would have been heavily influenced by medical theories of the time.
The New Bath Guide of 1778 mentioned "…a commodious cold-bath, with convenient dressing-rooms".
Behind-the-scenes tours, including the cold bath, will run on selected weekends in October.
