The largest prehistoric man-made cavern in the world may be hidden under a north Wales peninsula.
The mines are underneath Great Orme
The cavern is part of a Bronze Age copper mine complex which was first uncovered in 1987 at Great Orme's Head near Llandudno,.
Archaeologists excavating the 4,000-year-old site made their latest discovery 130ft below ground in December and have estimated it is at least 50ft in length.
They know the roof area is large, but will have to dig down through many layers of silt before they discover exactly how deep it is.
They have previously excavated four miles of tunnels at the complex, which is the largest Bronze Age copper mine in the world and is open to the public.
The first excavations began after the local authority surveyed the area in preparation for building a car park on top of it.
You never know what you're going to find, and that's the exciting thing
Tony Hammond, Great Orme Mines
Surveys indicate there are about 10 miles of tunnels in the area.
The largest known cavern before the latest one was discovered was the Great Stope, which is 60ft long, 25ft high and 25ft wide.
The copper mined at Great Orme by the ancient Britons would have been used to make axes and other implements.
Research showed that, initially, the copper was mainly used to make tools but, as time went on, more weapons were produced as people competed for food and land.
The caves could indicate tools were used in Wales long before the Romans came
Great Ormes Mines managing director Tony Hammond said it was "very, very early days" at present.
Excavation was only carried out during the winter time, when the mine was closed to the public, and work will not resume on the site until October.
"It's the second major cavern we have found. We located it after nearly three months of digging," Mr Hammond said.
"It's filled with rubble and rock.
"We have surveyed four miles of Bronze Age passages, and from time to time we find something.
"We're the largest copper mine from Bronze Age Europe. So this find will make it a bit bigger.
He added: "But you never know what you're going to find, and that's the exciting thing.
"It's really about December-time that we'll know more.
"From the size of the roof - some of which we can't see because it's blocked by silt - it does look as if it's going to be bigger."
Once the excavations have been completed, Great Orme Mines will officially present the find to heritage agency Cadw.