Archaeologists working on the site of a natural gas terminal in Pembrokeshire have uncovered what they believe may be a medieval metal works.
The team plans to make a full excavation now
The team was working at the site of the controversial liquefied natural gas (LNG) in Milford Haven when they found the works, which may date from 800AD.
Experts said little was known about this period, and the find could be a sign of early industrialisation.
Exxon Mobil, which is building the plant, said work would not be affected.
A team from Cambria Archaeology was at work on the site, which is standard practice whenever developers embark on a major project.
The group, led by archaeologist Peter Crane, found evidence of a furnace, charcoal and iron deposits as well as slag.
They came across the find by accident, and will now carry out more tests and a full excavation.
The find could help give an insight into how people lived at the period.
Mr Crane told BBC Wales: "It's getting the everyday activity but it'll also be the early industrial phase that is actually taking place on this site.
"We know that iron is important - I'm not saying this is an iron site, it could be bronze, there's swords being traded, there's a lot of metalwork around.
"But there's no pottery, which is our normal dating and activity indicator.
"This is really exciting because of the date which has come back from the radio carbon from the charcoal we found.
"It's a date of between 750-1100 AD which we know little of what's going on.
"We know there is activity, but (not) quite exactly what that detail is.
"It's slowly filling it in, and this is why we do such detailed excavation now, to try to understand the minutae of everyday activity rather than the top people in society."
John Constable from Exxon Mobil said the site was in its "third generation" of industrial activity.
"It looks like we had metal processing here perhaps 1,000 years ago.
"In the 50s the oil refining industry came, and now there we are in the 21st century at the start of a new era of natural gas import and processing," he commented.
The £250m site could see Milford Haven become the third busiest port in the UK. A deal was recently signed in Qatar in the Middle East to ship LNG to west Wales for 25 years.
However, protesters against the development have said not enough is known about LNG to make the site a safe option for local residents, and have cited applications in the US which were turned down because of safety concerns.
Pembrokeshire Council has granted planning permission for the site at the former Esso refinery and a Petroplus plant at nearby Waterton.