They can roll up stealthily out of calm seas, rogue waves up to 10 stories tall that have slam-dunked oil tankers, drowned beach-goers, swallowed ocean liners and crashed over the decks of aircraft carriers.
Now, a Johns Hopkins University scientist has begun to describe the shapes these rare, awe-inspiring swells can take, work that ship designers and oil-rig builders may one day use to anticipate the worst the sea can toss at them.
Owen M. Phillips, a Hopkins professor of earth science, and one of his former graduate students, Daifang Gu, have also discovered a way for weather forecasters studying a particular storm to predict the shape of the rogues that storm might spawn.
Until recently, little was known about them, because there is usually not much time to study them.
"When they hit a ship, people are looking after their skins," said Dr. Phillips, who has been studying the physics of waves since the 1950s.
These boat-shattering swells appear in all of the world's oceans, mostly in storms or squalls but sometimes in clear weather, throughout the year. There are dozens of accounts of ships they sank or damaged -- from the wave that capsized the 57-foot Fish-n-Fool off Baja California in 1987 to the rogue that battered the Queen Mary in 1942. Dr. Phillips said the U.S. Navy reports one or two rogues hitting its ships each year.
They range in size from 18- to 20-foot swells to the 112-foot mountain of water -- higher than the old IBM building on the north side of Pratt Street near the Inner Harbor -- that somehow spared a U.S. Navy ship, the Ramapo, in the North Pacific in 1933.
Even smaller rogues can threaten warships. In the pre-dawn hours of Halloween 1989, the carrier USS Dwight D. Eisenhower was cruising 90 miles southeast of Cape Hatteras when a rogue more than 20 feet high struck, catching a group of sailors on an open elevator between the flight and carrier decks.
Three sailors were washed overboard, along with 38 Sparrow and Sidewinder missiles weighing many tons. Two crew members were rescued, but the third was lost.
The Eisenhower incident helped inspire renewed interest in rogues by the Navy. Beginning in October 1990, the Navy and NASA conducted what they called the Surface Wave Dynamics Experiment, a six-month study using buoys and highly sensitive radar to study the ocean's surface off Wallops Island, Va.
Dr. Phillips and Dr. Gu, who now teaches at Princeton University, have studied data from the larger waves created during Atlantic storms. They found that the shape of a given storm's largest waves -- and presumably its rogues -- could be predicted by measuring the shape of the storm's smaller waves and plugging the numbers into a statistical formula.
The researcher says he has also found at least four shapes that rogues can take -- long walls, short walls, lumpy mounds and, the rarest of all, waves shaped like pyramids or volcano cones.
Dr. Phillips, who plans to publish his findings in the Journal of Physical Oceanography early next year, isn't sure how many other shapes he's likely to find. "We've found four very different shapes in the four storms we've analyzed," he said.
The shape of a rogue can make a big difference to a ship caught in one. "The most dangerous ones are not the ones with the highest crests, but those with the deepest troughs," said Dr. Phillips. "If you hit a high crest, it may roll across your deck and do some damage. But mostly it will wash over, and you'll be OK.
"If you hit a deep trough, then the water sort of drops away from the bow, you go nose down," he said. "Then you hit the wall of water on the other side, and your ship becomes a kind of submarine. . . and that can be fatal."
"If you know the shape of giant waves, then you ask what is the maximum slope that you will encounter," said Norden E. Huang of NASA's Goddard Space Flight Center in Greenbelt, the architect of the wave experiment. "Then you design for that kind of encounter."
Giant waves are sometimes generated by Atlantic storms moving south, opposite the flow of the Gulf Stream, off the Eastern Seaboard. That apparently happened July 3, when a rogue estimated to be 18 feet high, 27 miles long and 250 feet wide crashed onto Daytona Beach, Fla.
About 100 cars and trucks were sent tumbling, and some were sucked out to sea in the backwash. Seventy-five people were treated for minor injuries. The ocean off Daytona Beach was calm, but scientists later said the wave probably was triggered by a squall heading south off the coast of Georgia that evening.
Rogues have struck beaches near San Francisco, Hong Kong and the French Riviera. They are variously called "freaks," "sleepers," "sneakers" or "giants" by sailors and beach residents around the world.
Off South Africa's treacherous southeastern or Transkei coast, giant waves are known locally as "holes in the ocean." They appear where the Agulhas current, moving southwest, slams into storm winds and Antarctic currents heading in other directions.
Between 80 and 100 ships have been wrecked in that area, where giant waves from 25 to 30 feet high can roll behind troughs between 25 and 30 feet deep. The Greek cruise ship Oceanos sank off the Transkei in August 1991, after wallowing in 24-foot waves while storm winds gusted up to 88 mph.
South African sea captain C. J. Harris survived an encounter with a "hole in the ocean," and in 1991 described it to a reporter this way: "It was a Sunday afternoon and I was in my bunk when the ship just dropped into a hole. It was like going down in a lift [elevator] and then this huge wave crashed down on the ship and stripped all the lifeboats away."
In 1909, witnesses on shore said the British passenger liner Waratah was engulfed in a single wave and disappeared, in what the London Times has called "one of the mysteries of this century." All 211 people on board were lost. The Waratah is now believed to have been the victim of a giant wave. Its wreckage was located on the seabed in 1990.
Giant waves are suspected in the unexplained disappearances of several other ships, Dr. Phillips said. "If you hit one of these things, there's no time to send a message," he said.
Unlike tidal waves or tsunamis, giant waves are not triggered by earthquakes or volcanic explosions. When currents collide, the ocean stores the energy until a certain critical point, when the sea swells into towering walls of water.
Researchers think rogues are spawned even on relatively calm days. This happens when many of the hundreds of smaller sets of waves that normally jostle each other across the ocean's surface accidentally start to roll in unison, and merge into a few huge waves.
"If all the components come together at a certain place and all reinforce each other, you have a big wave," Dr. Huang said.
Scientists say it is possible to design ships to better withstand giant waves and to identify those sea surface conditions likely to spawn them. But, they add, weather forecasters
will probably never be able to predict where and when they will occur.
"It's like a tornado," Dr. Phillips said. "You can't tell exactly where a tornado will touch down. You can't tell where exactly these waves are going to appear."