Researchers discovered the North Pole was ice-free with tropical temperatures, about fifty-five million years ago.
In 2004, the Arctic Coring Expedition (Acex) used ice-breaking ships and a floating drilling rig to remove 400m-long cylinders of sediment from the bottom of the ocean floor.
The cores were taken from the 1,500km-long (930 miles) Lomonosov Ridge, which stretches between Siberia and Greenland.
The core holds layer upon layer of compressed fossils and minerals, which when studied can tell the story of millions of years of Arctic history.
The bottom end of the cylinder helped scientists to uncover what had happened to the Arctic during a dramatic global event known as the Palaeocene-Eocene Thermal Maximum, which occurred about 55 million years ago.
"This time period is associated with a very enhanced greenhouse effect," explained Appy Sluijs, a palaeoecologist from Utrecht University in the Netherlands, and the lead author on one of the papers.