By David Shukman
BBC science correspondent, Nazare
Scientists have begun the first detailed exploration of a vast underwater valley the size of the Grand Canyon - just off the coast of Portugal - and it has yielded a series of surprises.
Using Britain's ISIS robot submarine - a van-sized bundle of high-technology - researchers are for the first time able to view previously hidden features up to 5km (three miles) deep in the Nazare Canyon.
The canyon extends out into the eastern Atlantic from the seaside town of Nazare, north of Lisbon - long plotted on maps but until now never properly studied.
The submarine is operated from the UK's new research vessel, the James Cook, and during a visit on board I watched as it was winched over the side and lowered into the waves.
In a control room like something out of Star Wars, a team "flies" the robot down into the dark and the high-definition cameras have captured sights no one expected.
The Nazare Canyon teems with life - even at great depth
To the amazement of scientists, the shape of a shark appeared at a depth of 3,600m (12,000ft) - far deeper than sharks are usually found.
On the sides of the canyon, beneath the overhanging edges of the giant cliffs, cold-water corals are seen clinging to the rock, part of a highly active ecosystem well below the limits of sunlight.
According to the lead scientist, Professor Doug Masson of the National Oceanography Centre, the scientific community had been divided on whether "a canyon this deep would be a biological hotspot or an underwater desert".
In fact, he says: "It's a mixture of both - some areas like the walls are as active as a coral reef, while others are dominated by sand dunes with no signs of life."
And this undersea landscape is far more active than thought - giant boulders litter part of the sea-floor after being transported dozens of kilometres from the coast.
For Professor Paul Tyler, a marine biologist, the expedition is a chance to establish a baseline of data about this undersea world - so the effects of climate change can be assessed.
"We've seen signs of change at the surface and in other parts of the deep ocean at 5,000m; so we need to see what's changing here.
Climate change will impact even remote ecosystems
"There is nowhere on the planet that is immune from climate change."
The team's next mission? The Whittard Canyon, another deep submarine valley, this time off the coast of Ireland.
As Professor Masson puts it, less than 5% of the world's sea-bed has been surveyed with modern technology - so this project is just a start.