Workers redeveloping the site of a former steelworks near Wrexham have uncovered a fossil forest believed to date back 300 million years.
Geologists are now overseeing the excavation of the site, which is 50m long, at the old Brymbo works.
The forest pre-dated the dinosaurs and was from a time when what is now Wales was hot and humid and over the equator.
It is hoped the fossils can be conserved as a heritage attraction alongside new homes and industry.
The fossils date from the carboniferous period, before flowering plants.
Wales would have been dominated by tropical rainforests, whose compressed layers of decaying growth formed its coal deposits.
Developer Parkhill Estates is preserving the find using plastic sheeting and gravel but geologists hope in the long term a protective dome might be built as a heritage site.
The "trees" were not like trees at all, but a giant club moss, which grew to a size of up to 40 metres.
A protective dome is one idea to protect the fossils
Dr Jacqui Malpas, who is overseeing the excavation of the fossil site called the site extraordinary.
She said so far 20 fossilized trees had been uncovered, but others were being left undisturbed for fear the weather would damage them.
"It's rare to see so many fossilized trees," she said.
"The aim now is to catalogue all the finds and record the site. The forest is, however, threatened by rainwater, which is scouring the soft, protective mudstone. It's a race against time."
There are three other fossilized forests in Scotland, Dorset and Sheffield but the numbers found here are what makes it so interesting, as well as the timeline which links forward to the old iron works.
One idea is to recreate a tropical climate under a dome, complete with modern-day plants, with their ancient fossil relatives in the surrounding rock.
"There is wonderful potential to include the fossil forest within the former Brymbo works heritage area," added Dr Malpas.