Sharks come silently, without warning. There are three ways they strike: the hit-and-run, the bump-and-bite and the sneak attack. The hit-and-run is the most common. The shark may see the sole of a swimmer's foot, think it's a fish and take a bite before realizing this isn't its usual prey. It swims away, leaving the bleeding victim in need of stitches. The bump-and-bite is far more serious. Last year Chuck Anderson was training for a triathlon off Gulf Shores, Ala., when he was bumped by a bull shark, testing whether he was preyworthy. It decided that he was and then repeatedly attacked Anderson. He lost an arm.

Then there's the sneak attack. The shark is in the right place to find its prey, it is the right time to feed, and the target is the right size. At sunset on July 6 off Pensacola, Fla., Jessie Arbogast, 8, apparently fit the needs of a bull shark. Dusk is one of the shark's feeding periods; the boy was in the shallow water where the bull prowls; and splashing about, Jessie may have seemed to be a large fish. The shark pounced. The ensuing attack and the boy's struggle to survive have stirred an inchoate fascination — part fancy, part dread — with nature's sleekest predator.

Suddenly reports of shark attacks — or what people thought were shark attacks — began to come in from all around the U.S. On July 15 a surfer was apparently bitten on the leg a few miles from the site of Jessie's attack. The next day another surfer was attacked off San Diego. Then a lifeguard on Long Island, N.Y., was bitten by what some thought was a thresher shark. Last Wednesday a 12-ft. tiger shark chased spear fishers in Hawaii. News crews stood on the sand to interview experts, who declared over and over that sharks killed only 10 people worldwide in 2000. But don't swim at dusk or dawn; avoid murky water and steep drop-offs; shed all jewelry. And do swimsuits in yellow — "yum-yum yellow" — attract sharks? No one was sure. Sharks don't give interviews.

Shark attacks have been on the rise in recent years. But for all the terror they stir, the numbers remain minuscule. Worldwide, there were 79 unprovoked attacks last year, compared with 58 in 1999 and 54 the year before. Two-thirds were in U.S. waters. The higher numbers may reflect more surfers, boogie-boarders and open-water swimmers — more people splashing around, hence more attacks. Volusia County, Fla., holds the state record for attacks because its long coastline and many beaches are increasingly packed with bathers from the booming cities of central Florida.

Humans are much more dangerous to sharks, which tend to end up in soup or medicine. Fishing nets tangle and drown about 100 million sharks each year. In California there is only one shark attack for every 1 million surfing days, according to the Surfrider Foundation. You are 30 times as likely to be killed by lightning. Poorly wired Christmas trees claim more victims than sharks, according to Australian researchers. And dogs — man's best friends — bite many thousands more people than sharks do.

But these are terrestrial and mundane risks. Sharks lurk in the vast, mysterious ocean, an element that still stirs mythic fear. Science is shedding light on why sharks behave the way they do. Researchers are tracking sharks via satellites and coming closer to understanding why they attack humans. The three large sharks that account for most attacks on people — the great whites, the tigers and the bull sharks — have been studied extensively. We now know that great white sharks keep their blood warmer than the surrounding water, that tiger sharks do not return to the site of an attack to prey again, and that bull sharks have the highest levels of testosterone measured in any creature, land or sea. Each has a different diet, a different behavior pattern and a different mode of attack.

Scientists ultimately hope to de-mythologize sharks, to erase their images as rogue man-eaters like the great white shark that figures in Jaws, the Peter Benchley novel turned Steven Spielberg movie classic. Benchley, who says he is now "a full-time ocean conservationist," told Time last week, "I couldn't write Jaws today." After 25 years of research, the demonization of sharks doesn't hold, he says. "It used to be believed that great white sharks did target humans; now we know that except in the rarest of instances, great white shark attacks are mistakes." Dr. Robert Lea, a marine biologist working for the state of California, goes further: "I used to call them shark attacks — now I call them incidents. It is not a case of sharks preying on humans. It is just humans sharing a spot in the ocean with sharks — at the wrong time."

Sharks are one of nature's ultimate designs, tested over 400 million years — confident, sleek and lethal (see graphic). Studies show some sharks can measure changes in electric currents as tiny as five-billionths of a volt. They use this ability to hunt for prey hidden under the sand and to navigate according to the earth's magnetic field. "They are like some high-tech AWACs thing, with all their sensors," says Sean Van Sommeran, executive director of the Pelagic Shark Research Foundation in Santa Cruz, Calif. When they do attack a human, the weight of evidence now suggests,they have mistaken a person for a seal or some other prey, and most often will spit out human flesh after the first bite. The problem is, of course, that the one bite comes from jaws that are up to 3 ft. across and lined with hundreds of knives.

THE GREAT WHITE
Scott Yerby never saw the great white shark before it attacked him as he surfed off Clam Beach near Eureka, Calif. "This thing jumped me — it had enough force to lift me right out of the water. It was on my leg, I could see my femur, there was blood in the water — I knew then it was pretty serious," says Yerby, who was 29 at the time of the August 1997 attack. He hit the shark on the nose (the prescribed last-ditch defense, along with ripping at its gills), managed to get back on his board, and with his surfing buddy, paddled back to shore. By the time an ambulance got him to the hospital, doctors said, he had lost almost half the blood in his body and was close to death. As he recovered from his wounds in the hospital, many of Yerby's visitors asked him if he intended to go out and hunt down the shark that attacked him. "I said I had no reason to — he was in his element," says Yerby.

The great white is perfectly adapted to its element. Sometimes growing to more than 20 ft. long and up to 4,000 lbs., it keeps its body temperature 5šF to 10šF higher than that of the surrounding water by recycling heat from its swimming muscles. This allows great whites to hunt in cooler seas. "It seems to make them more vigorous," says Van Sommeran. The sharks are voracious eaters of seals when they patrol the Red Triangle — a 100-mile strip of California coast from Bodega Bay to Santa Cruz. They have enormous livers to store energy, and can go for months without eating. Nobody has seen great whites mate, but some biologists theorize that after fattening up off the coast, they head into the deep to procreate.

Van Sommeran's team, collaborating with the Point Reyes Bird Observatory and the Hopkins Marine Center at Stanford University, is on the verge of finding where the great whites go after they leave the coast of California in the winter. Since 1999, the biologists have been attaching "pop-up tags" to great whites. These continually measure the shark's position, depth, speed and direction, and store the data in digital archives. After six months, the tiny computer in the tag sends an electric current through a magnesium burn wire, which dissolves in the seawater and allows the tag to pop up to the surface. The tag transmits a GPS locator signal, and when satellites get a fix on it, they upload all the archived data of the shark's movements. The distances are likely to be huge. A shark tagged in Australia in a similar experiment this year traveled more than 1,800 miles along the country's coast in three months.

Great whites are the most lethal to humans. Since 1876 there have been 254 confirmed nonprovoked attacks on humans by great whites, 67 of which were fatal, according to statistics compiled by the International Shark Attack File at the Florida Museum of Natural History. Over the same period, tiger sharks have attacked 83 times with 29 fatalities, and bull sharks have attacked 69 times with 17 fatalities. Great white attacks on humans generally involve just one bite. Researchers are not sure, but most think the shark's sensory organs quickly differentiate between humans and the blubber-rich seals it prefers, so it effectively bites and spits out humans.

Researchers who have been observing great whites off the Farallon Islands west of San Francisco think they know why sharks mistake humans for seals. Peter Pyle, a biologist for the Point Reyes Bird Observatory, notes that the majority of great whites that attack humans are in the 8-to-12-ft. range — inexperienced juveniles making the diet transition from fish to bigger, more nourishing seals. "They are learning a new hunting technique and may mistake surfers for seals," says Pyle. Once the sharks get bigger and more experienced, they appear better able to differentiate between seals and humans.

Unlike tigers and bulls, great whites hunt mostly during the day, and their preferred method of attack is to shoot up vertically from 30 ft. down, knocking their prey right out of the water with the impact. Researchers in South Africa have produced spectacular footage of great whites leaping 15 ft. into the air with a seal in their teeth.

TIGER SHARK
esse Spencer, now 18, from the Big Island of Hawaii, was surfing near Kona in October 1999 when a 10-ft. tiger shark came halfway out of the water and pushed him off his board. The shark's nose struck Spencer's head, then its jaws locked onto his arm. "I could almost see the whole shark. My elbow was down his throat." The shark ripped muscles, tendons and blood vessels, then chomped down on the surfboard before finally disappearing. Spencer made it to shore, and today his arm is recovering, although he still cannot grip with his hand. His mistake? Surfing at sundown.

The tiger shark generally hunts at night. It is an indiscriminate eater, "willing to try anything for food," says Rocky Strong, a shark biologist associated with the Jean-Michel Cousteau Institute. Not just fish, turtles and sea mammals but also dogs, boots, beer bottles and unopened cans of beans. Its teeth are serrated, with a notch to catch and cut through ligament or shell tissue.

Tiger attacks on humans have been on the increase in Hawaii, and one reason, says John Naughton of Hawaii's Habitat Conservation Program, may be the increase in seagoing green turtles since they were protected in the 1970s. "Turtles come close to the shore, and the tigers follow them to prey on them. That puts them in the same area as swimmers and surfers." Tigers are slower swimmers than great whites and not as good at surprise. Human victims often see the shark before it closes in to attack. But tigers are persistent. "If you are bitten by a tiger, you have a good chance of being chewed up. They come back," says John McCosker, a scientist at the California Academy of Sciences.

After a spate of attacks in Hawaii in the early '90s, islanders headed out to kill the rogue tigers. But scientists have since learned that tigers are not territorial, and so chances of catching the culprit at an attack site are minimal. Dr. Kim Holland of the Hawaii Institute of Marine Biology at the University of Hawaii has been monitoring tiger movements with the CHAT (Communicating History Acoustic Transponder) tag. Implanted in belly walls to log the shark's position and depth, the CHAT tags upload their information to underwater receivers, usually placed in shallow bays, which are retrieved every three weeks. "We know they don't stake out declared territories. They are inter-island travelers," says Holland.

BULL SHARK
When Dawn Schauman was attacked by an 8-to-10-ft. bull shark in October 1993, she said, "it felt like a truck had slammed into me, then I felt a compacting squeeze and an acute burning in my left hand and my left leg." The shark spun her around, leaving her disoriented as she hemorrhaged blood into the water. The shark left, and willpower alone got Schauman — 6 1/2 months pregnant — back to shore. Her baby was later born prematurely but safely. For months Schauman woke at 3 a.m. replaying the attack in her head.

The bull shark, Carcharhinus leucas, usually grows no longer than 10 ft. and weighs up to 500 lbs., but what it lacks in size it makes up for in aggressiveness. Experts regard it as the most pugnacious of sharks. It has, according to Robert Hueter, director of the Center of Shark Research at the Mote Marine Laboratory in Sarasota, Fla., the highest level of testosterone in any animal, including lions and elephants. Its lower spiked teeth are designed to hold prey while the upper triangular serrated teeth gouge out flesh. "The bull is an ambush type of predator, it makes this big mortal wound," says Hueter. It is fearless, taking on prey as large as it is.

A unique feature of bull sharks is their ability to live in both salt- and fresh water; they have attacked people in Lake Nicaragua in Central America and have been seen above St. Louis, Mo., in the Mississippi River. Those born in the Mississippi delta usually spend about six months in the brackish water before migrating along the coast to Florida to winter in the Keys.

The bull is the only shark that prowls regularly in water shallow enough for humans to walk in — and it may be territorial. Australian shark biologist Ian Gordon has been getting into the water off Florida beaches and deliberately agitating bull sharks to observe their reaction. He says his research so far suggests that underwater geography and a sense of territory can provoke an attack. "Even if you don't know it's there, the shark will feel like it is being cornered."

Human shark victims almost always seem to be inadvertent intruders rather than targeted prey. Scientists who work with sharks know how dangerous they can be, and many are critical of the guided shark-feeding tours that are proliferating in Florida and the Bahamas. Sharks there have begun to associate the sound of an outboard motor with food, and there have been attacks by sharks apparently impatient to be fed, according to George Burgess, head of the International Shark Attack File. Shark feeding is illegal in two Florida cities, and a campaign to ban it statewide is under way. "When you are training animals, you are changing their basic behavior and their respect for human beings," says Burgess.

That would be a strange development: the ocean's fearsome hunters lured unnaturally into the company of humans — then learning to bite the hands that feed them. Nature has its bounds.

— Reported by Alice Jackson Baughn/Ocean Springs, Paul Cuadros/Gainesville, Lisa Clausen/Melbourne, Jeanne DeQuine/Bahamas and Jeannie McCabe/Honolulu