Scientists say its appearance was caused by a sudden upwelling from the Earth's mantle, the BBC reported Thursday.
The undersea river-valley system is to the northwest of Scotland, about 125 miles west of the Shetland Islands.
The researchers reflected sound waves off 3,800 square miles of seabed to discover the new landscape.
"It is supposed to be just boring layers of mud and sand down there," Nicky White of the University of Cambridge said.
"What we found was a big surprise: three-dimensional surfaces of hills and valleys."
It looked just like the surface of land, the researchers said, except it was under 1,000 yards of water and a further 2,000 yards of accumulated sediment.
The team managed to obtain samples of terrestrial plants and rotting vegetation that proved this had once been land.
"This hidden landscape was above the waves between about 56 and 55 million years ago," White said.
"It must have risen about a kilometer within 2 million years and then subsided just as quickly. That is extraordinary," he said.