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This photo released Monday by the Illinois State Geological Survey shows a fossil, part of a fossilized rain forest discovered in coal mines in Vermilion County in eastern Illinois. ((Illinois State Geological Survey/Associated Press))

Coal miners working south and west of Georgetown, Ill.,have unearthed, chunk by chunk, what has revealed itself over the past few years to be the remains of a fossilized rain forest from the Pennsylvanian Period, 300 million years ago.

It covers about 25 square kilometres, all more than 60 metres below ground, and probably is the largest intact rain forest from that period ever studied, according to Scott Elrick of the Illinois State Geological Survey. It's that scale that makes what lies just above the Riola and Vermilion Grove mines significant, he said.

'We've never encountered one whole forest preserved in one shot like this.' —Scott Elrick, Illinois State Geological Survey

"We've never encountered one whole forest preserved in one shot like this," Elrick said Monday. "The fossils just didn't stop."

It's common to find small pockets of fossilized plants just above coal mines, he said. But in this case, experts believe, a fault that runs through the area unleashed a major earthquake that quickly sank the forest beneath a deep layer of mud, preserving it.

Fossilized time capsule

"What they're looking at is very rapid preservation of this forest," meaning that plant tissue was preserved in great detail, rather than being broken down over time, said Ian Glasspaul, a collections manager at the Field Museum who is not involved with the work in Vermilion County.

"It's a snapshot in time," he said. "That's what makes it exciting."

Elrick and researchers from the Smithsonian Institution and the University of Bristol in Great Britain started working in the mines a few years ago, driving deep underground in armoured vehicles and then walking along kilometres of two-metre-high passages.

They spent most of their time looking up, according to Howard Falcon-Lang, the scientist from the University of Bristol. That's because the coal that's being mined used to be the soil that the ferns, mosses and trees of the rain forest grew on, he said in an e-mailinterview.

Coal seams found across the United States, the United Kingdom and Europe once were the soil beneath the first rain forests, he said.

Illinois forest was a jungle

People who live in eastern Illinois may occasionally long for a few more trees, but they'd find the land that now sits just above the miners' heads a tough place to call home during the Pennsylvanian Period, Elrick said. The plants were bigger— 10-metre-tall horsetails and mosses as big as trees— but familiar enough. The heat and humidity would be something else entirely.

"It would be hot, extremely humid, really uncomfortable to be standing around there," Elrick said. "Something out of Sir Arthur Conan Doyle's Lost World."

Researchers haven't found much evidence of animal life, but they have found a eurypterid, a two-metre-long lobster-like creature that would have crawled from beneath the waves of the long-gone Absoroka Sea, Falcon-Lang and Elrick said.

Derrel Carter, a spokesman for Peabody Energy, said mining has stopped at the Riola mine but continues at the Vermilion Grove site.

Elrick and the other researchers plan to continue documenting what's above the Vermilion County mines, drawing and taking pictures and notes. But that's all they'll do, he said. The area deep underground isn't suitable for preservation.

"Unfortunately, it will never be a visitable museum kind of piece," Elrick said. "We try to document to the best of our ability what we see, and take notes…. It's sort of like asking people to go to New York City and describe every storefront in a day."