BBC News writes scientists have found a spectacular fossil forest in the coal mines of Illinois. The more than 300 million year old fossils are 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.