Suspended in the blackness of the deep, teeth glinting in floodlights, the robotic jaws descended towards their target. Watching from the control room more than half a mile above, on board RRS Discovery, technicians were expecting their new robot to explore nothing more than a sediment-covered hill. Instead, the live video feed revealed the jagged peaks of a recently erupted volcano, adorned with a quivering, silver forest of eel-like fish.

The Casablanca Seamount lies 300 miles (480km) west of Morocco, at the eastern edge of a chain of undersea volcanoes that formed the Canary Islands and was thought to have been dormant for 15 million years. That was until the National Oceanography Centre in Southampton chose the 2½-mile-high mountain as the test site for the new £200,000 Hydraulic Benthic Interactive Sampler (HyBIS).

Looking through a camera mounted inside its jaws, operators can adjust its position from the control room. As they closed in on the never before seen mountain, they were in for a shock. “We were expecting to find it completely covered in 15 million years’ worth of mud,” Bramley Murton, the expedition’s chief scientist, said. “Instead, we found lots of bare rock, fresh lava flows on its flanks, and lots of small crater pits — evidence of fluid flow.”

The sight testified that the volcano had seen activity within the past 10,000 years, which by geological standards made it a recent event, Dr Murton added. But he was in for a bigger shock when the dangling robotic jaws rounded an undersea cliff more than 700 yards down.

“We came upon a huge field of eels, each two metres long, standing on their tails, quivering in the current — there were hundreds of these things,” he said. “They looked like a birch forest in winter.”

The description appears to match that of the silver scabbard-fish, Lepidopus caudatus. It is a resident of Atlantic seamounts but has never been recorded this deep. The scabbard-fish’s vertical posture is thought to camouflage it from predators who hunt by looking for silhouettes above, while it also keeps them poised to strike themselves.

Professor Imants Priede, of the University of Aberdeen, said: “Whatever they are, it is an amazing discovery, seeing fish like this for the first time. Most of what we know of deep-sea fish is derived from analysing dead specimens. Living fish showing natural behaviour is a wonderful thing.”

Apart from the silver forest of fish, the seamount was sprinkled with cold-water corals, sponges and sea cucumbers, and patrolled by lobsters and sharks. “It was absolutely amazing,” said Viet Huehnerbach, the HyBIS co-pilot. “I’ve been on 50 cruises and I’ve never seen anything like this before.”

The unexpected signs of recent volcanic activity and the forest of eels were not the only revelation for the scientists. “Given we were in the mid-Atlantic, hundreds of miles from anywhere, there was a surprising amount of rubbish; beer bottles, tin cans. We also saw trawlmarks and fishing lines with floats, even at six or seven hundred metres,” Dr Murton said.

The HyBIS is designed to scoop samples of up to half a cubic metre from the seabed, keeping them protected within its jaws until they are opened on board the ship above. It will be sent next to the Mid-Cayman Rise in March to explore hydrothermal vents.

Explore UK News

• Crime News

• Education News

• Health News

• Scotland News

Times Recommends

• Foxy Knoxy snared by her lust and her lies
• Britain is the land of the shrinking family
• Zac Goldsmith: Not such a green god