World's largest fossil forest found in Illinois coal mine

Illinois geologists have discovered the remains of one of the world's oldest tropical rainforests, preserved in the ceiling of a coal mine 250 feet below the surface. The fossil forest encompasses four square miles, which is the largest find ever.

Plant fossils are common in coal beds, as coal is the compacted result of peaty plant material. But scientists are surprised by the size of this fossil bed, which they suspect came about because of a freak event: an earthquake that flooded and buried the forest. The vast extent of the fossil forest, which existed in a swampy time of giant dragonflies and tree ferns, has allowed the scientists to infer subtle ecological changes across the ancient landscape.

These are the remnants of extinct plants from a geological period 300 million years ago, called the Carboniferous, when the world was covered in a riot of green. Illinois was near the equator and much warmer and wetter. It was also a time before flowering plants had evolved, and so the plants would seem bizarre to modern eyes.

Giant tree ferns would have formed a lower canopy 30 feet high. Poking up through the ferns would have been 100-foot-tall clubmosses — asparagus-like poles that sprouted crowns full of spores. It was the age of insects, with 6-foot-long millipedes and dragonflies with yard-long wingspans.

When the ancient earthquake hit, a sudden flooding in the submerged block killed the rainforest. Mud and silt rushed into the depression, preserving the stumps and logs in a layer that eventually became shale.