By Jonathan Amos
Science reporter, BBC News, Liverpool
The remains of a tree that grew about 300 million years ago
Spectacular fossil forests have been found in the coal mines of Illinois by a US-UK team of researchers.
The group reported one discovery last year, but has since identified a further five examples.
The ancient vegetation - now turned to rock - is visible in the ceilings of mines covering thousands of hectares.
These were among the first forests to evolve on the planet, Dr Howard Falcon-Lang told the British Association Science Festival in Liverpool.
"These are the largest fossil forests found anywhere in the world at any point in geological time," he told reporters.
"It is quite extraordinary to find a fossil landscape preserved over such a vast area; and we are talking about an area the size of (the British city of) Bristol."
The forests grew just a few million years apart some 300 million years ago; and are now stacked one on top of another.
It appears the ancient land experienced repeated periods of subsidence and flooding which buried the forests in a vertical sequence.
They have since become visible because of the extensive mining operations in the border area between the states of Illinois, Indiana and Kentucky.
Scientists look up to see what would have been the forest floor
Once the coal seams have been removed (what were, essentially, the compacted soils of the forests), it is possible to go into the tunnels and look up at what would have been lying on the forest floors.
"It's a really exciting experience to drive down into these mines; it's pitch black," the Bristol University research said.
"It's kind of an odd view looking at a forest bottom-up. You can actually see upright tree stumps that are pointed vertically up above your head with the roots coming down; and adjacent to those tree stumps you see all the litter.
"We found 30m-long trunks that had fallen with their crowns perfectly preserved."
Some of the preservation is exquisite
The researchers believe their study of these ancient forests could give hints to how modern rainforests might react in a warmer world.
The six forests straddle a period in Earth history 306 million years ago that saw a rapid shift from an icehouse climate with big polar ice caps to a greenhouse climate in which the ice caps would have melted.
"The fascinating thing we've discovered is that the rainforests dramatically collapse approximately coincident with the greenhouse warming," explained Dr Falcon-Lang.
"Long-lived forests dominated by giant club moss trees almost overnight (in a geological sense) are replaced by rather weedy fern vegetation."
The next stage of the research is to try to refine the timings of events all those years ago, and work out the exact environmental conditions that existed. The thresholds that triggered the ancient collapse can then be compared with modern circumstances.
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