Geologists have discovered a vast new landscape that rose above the north Atlantic waves 56 million years ago.
It was caused by a sudden up-welling from the Earth's mantle and may explain rapid climate change that took place at about that time.
Discovered by UK researchers, the river-valley system is off the north west of Scotland, about 200km west of the Shetland Islands.
The results are published in the journal Nature Geoscience.
Over a six-month period the researchers reflected sound waves off the seabed to discover the new landscape.
They surveyed about 10,000 sq km of seabed; the sound waves reaching depths of up to several kilometres.
"It is supposed to be just boring layers of mud and sand down there," said Nicky White from the University of Cambridge, UK who was involved in the project.
"What we found was a big surprise: three-dimensional surfaces of hills and valleys," he told BBC News.
The lead author Ross Hartley, from the University of Cambridge, UK, plotted out the surfaces and discovered clear evidence of the drainage pattern of a river system, going down to what looked like a coastline.
It looked just like the surface of land, except that it was under a thousand metres of ocean, and a further 2000m of accumulated sediment.
The team managed to obtain drill samples; not complete cores that might contain macroscopic fossils, just chips from commercial drilling. But that was enough to prove that this had once been land.
In the sediments above and below there were the microscopic remains of plankton such as foraminifera. But at the suspected land surface they found pollen from terrestrial plants and lignite from rotted vegetation.
"This hidden landscape was above the waves between about 56 and 55 million years ago," said Dr White.
"It must have risen about a kilometre within 2 million years and then subsided just as quickly. That is extraordinary," he added.
In a second paper, Dr White proposes how that might have happened.
Sea level changes are not nearly enough to account for it, he thinks. It must be due to the slow but persistent upwards convection of mantle rock from great depths. It is what makes Iceland volcanic.
Fifty-six million years ago there must have been a sudden pulse in that plume. The material was not hot enough to result in partial melting and huge volcanic eruptions.
Instead, it spread out like ripples on a pond beneath the lithosphere, the 100km thick slab of hard rock that makes up the Earth's crust and the top of the mantle.
It may not have been an isolated event. There is evidence of other land surfaces higher up in the sediment column, though none seems quite as extensive as the landmass that rose from the Atlantic 56 million years ago.
That is so large that its edges disappear off the 10,000 square kilometre survey area.
Back then the climate was much warmer, and the boundary 55 million years ago between the Paleocene and the Eocene periods is marked by what is known as a thermal event - a very rapid rise in global temperatures accompanied by a rise in atmospheric carbon.
One explanation for the carbon is the sudden release of methane held as ice in gas hydrates on the ocean floor. The drop in pressure due to sudden uplift could explain its release.
The sudden rise of the range across the north Atlantic would also disrupt ocean circulation and currents, which transport heat around the planet, so that to would have had a significant effect on climate.
There is evidence on the sea floor of repeated pulses from the mantle plume so, say the researchers, it could happen again, but on timescales of millions of years.
They describe their results on the BBC's Material World on Radio 4 today. (14th July at 16:30 BST)