A team of archaeologists have found "hidden" remains of prehistoric buildings and fields on Skomer Island, off the Pembrokeshire coast.
Using new technology, they "X-rayed" fields and found buried ditches and structures not visible on the ground.
Dr Toby Driver from the Royal Commission on the Ancient and Historical Monuments of Wales said they may date back 5,000 years.
He added they are among the best preserved anywhere in the UK.
Skomer Island is a national nature reserve and famed for its bird life and puffins.
It also has remains of prehistoric houses and much of the island has been designated an Ancient Monument.
But nobody had carried out any archaeological studies of the island since the 1980s.
Last April airborne laser scanning was completed of the island, which provided a model of the surface of landscape, including its houses and fields.
However, because the centre of Skomer was farmed until the 1950s, anything of interest on the surface was ploughed away.
So archaeologists from the Royal Commission and Sheffield University went to the island last week to carry out a geophysical survey, which uses technology to measure through the earth, creating an "X-ray" picture of what is under the ground.
It was the first time the technique had been used on the island and Dr Driver, who was part of the team, said it gave greater insight into what life on the island was like in the past.
"People had looked at this about 20 years ago and concluded it wasn't a very complicated landscape," he said.
"[They thought] people maybe lived there for 100 years and then moved back to the mainland.
"Work over the last three to four years by the Royal Commission has begun to demonstrate that actually we think people were there for a few thousand years rather than a few hundred years.
"From before the Roman times maybe back four of five thousand years before now.
"So it's a very busy landscape and we have been trying to investigate this further."
He added: "We now know that the centre of the island has been occupied in the Iron Age and possibly before the Romans and that means pretty much the whole island would have been a very busy place for two to three thousand years.
"We're very lucky on Skomer, it's a gem for Wales. It hasn't been ploughed around the edges - it hasn't been ploughed since the Romans left Wales, so that's about 2,000 years old.
"It's very, very well preserved. This is a place on Wales where we can study prehistoric fields, Iron Age life, a Celtic way of life , in a way that in other parts of Wales has been lost."
The team now hopes to do more work on the island in the future.