A rainforest represents an explosion of life, the peak of biological diversity on Earth.
But the trees that make the current rainforest appeared for the first time during the Cretaceous, at the sunset of the dinosaur realm. So, when did the first rainforest appear? And how did it look like?
Now a British-American team led by Dr Howard Falcon-Lang from the University of Bristol, UK, found in the underground workings of a coalmine, in Illinois, USA, the world's oldest rainforest: it is 300 million years old, from the Carboniferous period, when world most and best coal's resources formed ("carboniferous" = bearing coal).
The plants that made it are all gone now: club mosses (a type of fern), over 40 m (133 ft) tall, covering a sub-canopy of tree ferns and tree-sized horsetails (another type of fern).
Their descendants are all now tiny herbs.
A major earthquake 300 million years ago sank the whole area under sea level and the marine sediments that covered the forest fossilized it.
"It was an amazing experience. We drove down the mine in an armored vehicle, until we were a hundred meters below the surface. The fossil forest was rooted on top of the coal seam, so where the coal had been mined away the fossilized forest was visible in the ceiling of the mine. We walked for miles and miles along pitch-black passages with the fossil forest just above our heads. We were able to make a map of the forest by the light of our miner's lamps." said Falcon-Lang.
This is also the largest fossil forest ever discovered: it has over 10,000 hectares, 10 x 10 km (6x 6 mi).
"As there is nothing like it around today, before our work we knew very little about the ecological preferences and community structure of these ancient plants. This spectacular discovery allows us to track how the species make-up of the forest changed across the landscape, and how that species make-up is affected by subtle differences in the local environment." added Falcon-Lang.
Earth's first rainforests were highly diverse in tree species and changed across the ancient landscape.