The Azolla event is a dramatic paleoclimate episode that occurred approximately 49 million years ago during the middle Eocene epoch. At the time, the Arctic Ocean was a warm, landlocked basin with a stratified freshwater surface layer. Under these conditions, the fast-growing freshwater fern Azolla bloomed in massive mats across the basin. As the ferns died, they sank to the anoxic sea floor, trapping carbon in sediments without decomposing. Spanning over 800,000 years, this process sequestered massive amounts of carbon dioxide, reducing atmospheric CO2 levels from around 3500 ppm to 650 ppm. This drawdown triggered a global transition from a tropical 'greenhouse' climate to the 'icehouse' state that eventually allowed ice sheets to form at the poles.