Horrified colleagues from the British Antarctic Survey (BAS) scrambled a rescue boat and managed to pull Brown from the water. Despite trying to resuscitate her for an hour, the station doctor was unable to save the 28-year-old.

The BAS has now launched a full investigation into the incident.

Linda Capper, spokesperson for the BAS, said, "This was a completely unprecedented event. Kirsty is the first person we know of to have been killed by a leopard seal. We don't know why she was attacked.

"Our scientists have been diving and snorkeling in Antarctica for over 30 years and we've never experienced anything like this."

However, scientists are worried that increased human activity in Antarctica could lead to more life-threatening encounters with leopard seals.

Ian Boyd, director of the Sea Mammal Research Unit in Scotland, said, "There may be a process going on in Antarctica where these animals, because of their growing exposure to man, are becoming a greater danger."

Leopard seals rank alongside killer whales as Antarctica's top predator. Named after their spotted coats and fearsome jaws, leopard seals have large, reptilian heads and streamlined bodies. They propel themselves using powerful fore-flippers, reaching speeds of 25 miles per hour (40 kilometers per hour). Females grow larger than males, weighing up to 990 pounds (450 kilograms) and measuring over 13 feet (4 meters) in length.

Penguin Prey

The varied diet of these seals is reflected in their unusual tooth structure. As well as having large canines for dealing with bigger prey, their cheek teeth are serrated, which enables them to strain krill from the water. They also eat fish, penguins-and other seals. They tend to target crabeater and fur seals, but also juvenile Weddell, Ross, and southern Elephant seals.

Leopard seal feeding behavior is most easily seen when preying on penguins. The captured bird is thrashed about on the water until its skin peels away. The remaining carcass is then eaten.

Despite being at the top of the Antarctic food chain, relatively little is known about leopard seals. They are solitary animals, spread out over millions of square miles of pack ice. This makes them difficult to study.

Boyd said, "As with any top predator, like tigers and polar bears, leopard seals are charismatic creatures, but that isn't sufficient to justify the extremely high costs of working with them. They don't occur in very large numbers and finding them is difficult."

Boyd believes there are three possible reasons why a leopard seal would grab a diver. He says it could be a defensive reaction, having been startled, or a case of mistaking the diver for prey while hunting other seals.

A third possibility is more worrying. He said, "Leopard seals have been known to stalk people and it's possible an animal could attack a person while knowing exactly what it's attacking."

Leopard seals are regarded as inquisitive animals, but Boyd says divers should be wary of letting themselves become objects of curiosity.

He said, "I think this inquisitiveness towards humans is to do with sizing them up as potential prey."

And with more people going to Antarctica each year, he believes even greater care is needed. "Increasing numbers of people and increased diving activities mean these animals are coming into contact with man more often," he said. "So the seals are getting more opportunities to assess whether we're something they want to feed on."

Previous Attacks

Although attacks on humans are rare, they have occurred before. In 1985, Scottish polar explorer Gareth Wood had a lucky escape while walking across a thin ice layer.

In his written account of the encounter, he recalled, "Suddenly, the surface erupted as the massive head and shoulders of a mature leopard seal, mouth gaping in expectation, crashed through the eggshell covering. It closed its powerful jaws around my right leg, and I fell backward, shocked and helpless."

Wood was saved by his companions who repeatedly kicked the seal in the head with ice crampons until it released him.

Leopard seals have also been recorded attacking inflatable boats. United States Antarctic researchers had to fit special protective guards to prevent their boats being punctured.

Linda Capper says BAS scientists are aware of such dangers and always take special precautions.

"We have a lot of risk assessment and safety procedures in place," she said. "If a leopard seal is in the water then researchers don't go in. And if one approaches them or is seen while they're working in the water then the advice is that they come out."

Brown, who was an experienced diver, was researching the impact of iceberg scouring on marine life at the time of the incident.

Chris Rapley, director of the British Antarctic Survey, said, "This is tragic and shocking. My heart goes out to Kirsty's family and her colleagues at Rothera. Kirsty was a vibrant, dynamic individual, committed to her science and with a promising scientific career ahead of her."

In addition to the BAS's own investigation, the coroner for the British Antarctic Territory will carry out an inquest into Kirsty's death.