An international scientific team which has been drilling beneath the bed of the Arctic Ocean says it enjoyed a sub-tropical climate 55 million years ago.
The Arctic Coring Expedition (Acex) has recovered sediment cores from nearly 400m (1,300ft) below the sea floor.
It says fossilised algae in the cores show the sea temperature was once about 20C, instead of the average now, -1.5C.
The expedition, which has relied on three icebreakers during its work, is now heading back to Tromso in Norway.
The scientists, from eight nations, recovered the cores from below the sea floor in waters 1,300m (4,260ft) deep.
Acex has been taking cores from the Lomonosov Ridge between Siberia and Greenland. The ridge, 1,500km (930 miles) long, rises to 800m (2,625ft) below sea level and is topped by 450m (1,475ft) of
layered sediments.
The scientists said before they set sail from Tromso last month their findings would help science to work out how long the Arctic sea ice, now in retreat, had persisted.
The cores they have extracted show the Arctic Ocean was once a subtropical, shallow sea. The evidence, Acex says, is in the form of tiny algal fossils found in the cores, which were once marine plants and animals.
They date back to a period known as the Palaeocene-Eocene thermal maximum, a brief period that occurred around 55m years ago.
It was characterized by an extremely warm climate that created a natural greenhouse effect, which caused massive amounts of carbon to be deposited in both sea and air.
Atmospheric carbon levels then are thought to have been about 23,000 parts per million (ppm), compared with almost 380 ppm today.
The algae found in the Lomonosov cores, which lived only in subtropical conditions, prove how warm the Arctic once was, Acex says. It says the ocean's temperature was once similar to the waters off New York in August.
Dr Michael Kaminski, a palaeontologist from University College London, UK, said: "We're seeing a mass extinction of seabottom-living organisms caused by these conditions.
"Moving forward in time, we see many species disappear. Only a few hardy survivors endure the thermal
maximum."
There is also evidence that part of the Arctic Ocean was once a freshwater lake, probably when the Lomonosov Ridge was part of what is now Siberia.
The last 250,000 years of Arctic history were known already in some detail thanks to cores taken from the Greenland ice cap.
But Professor Jan Backman of Stockholm University, one of the two chief scientists of Acex, said: "We now have sediment records going back to 56m years, which are resting on 80-million-year-old bedrock.
"The early history of the Arctic Basin will be re-evaluated based on the scientific results collected on this expedition."
Acex has had to contend with natural hazards, including an ice shelf up to 10m (33ft) thick which threatened drilling operations before a Russian icebreaker demolished it.
The drilling ship was also approached by two polar bears, capable of climbing over its low sides, and had to scare them off by sounding its hooter.
The Acex scientists are to meet again in November at the University of Bremen in Germany to examine the data.
Acex is part of the Integrated Ocean Drilling Program (IODP) and is conducted by the European Consortium for Ocean Research Drilling.
A group of European scientific institutions, Ecord Science Operator, is responsible for fleet management, ice and weather monitoring, and science operations.
The British Geological Survey co-ordinates Ecord Science Operator, and the Natural Environment Research Council is a member of IODP.