We crowded into two boats and left the dock as day was breaking. It took us only 30 minutes to approach Seal Island, and we smelled it long before we saw it. Some 84,000 fur seals make their home here, and they covered every inch of the barren rock, barking, lounging, squabbling, and sliding clumsily into the water, where they metamorphosed into creatures of sleek and sinuous beauty.

Behind the boat we trolled a plastic seal with a videotape camera mounted in its belly, pointing downward.

We settled down to wait, confident that it would be hours before we.…

The water erupted. A gigantic body blasted through the surface—white beneath, gunmetal blue above, glinting in the dawn sunlight—with the seal decoy in its jaws. The shark flipped completely over, hit the water with a tremendous splash, and disappeared.

Before anyone could speak, it happened again: a rush from the dark, an explosion at the surface, a balletic somersault, a splash.

“The violence!” Rodney said. “My lord, the violence! We know they attack from below. We know they eat seals. But the violence!” He paused, then said, “If I’d seen this behavior soon after my attack, I tell you, today I’d be a first-class golfer.”

It was the breaching that none of us had ever seen, the sheer power of the attack that heaved the huge body out of the water. I turned to Rob and asked, “Does this happen often?”

“Every day,” he said. “Early in the day and late, morning and evening, when the seals leave the island to feed and when they come back. There’s.…”

Another shark soared into the air. This time the seal in its jaws was not one of our decoys.

“As I was saying,” Rob resumed with a wan smile, “there’s what you could call a ring of peril about a hundred yards wide around the island. The sharks patrol it. The seals know it.”

Indeed, as we watched, half a dozen seals slid off the rock and gathered at the surf line. All at once they disappeared. A few moments later they reappeared, but not together, each on its own a couple of hundred yards out in the water … outside the ring, I supposed.

No one knew why these great white sharks breached during the hunt, while the sharks Rodney had studied in Australia for 35 years didn’t. I could not recall hearing about populations of white sharks that breached so regularly and ferociously, nor could Rocky Strong, a young doctoral candidate who had joined us a few days earlier to work on a National Geographic Society grant to study white shark behavior.

Rocky did, however, find the breaching to be a possible cause for concern. “Each unsuccessful breach consumes a tremendous amount of energy,” he said. “If there’s a lot of debris on the surface, these guys could conceivably wipe themselves out chasing shadows.”

Two hours farther up the coast, in Gansbaai, shark-watching and cage-diving operations contribute mightily to the economy. Like False Bay, Gansbaai has a large seal colony close to shore, on Geyser Island, and in a channel around the island that has come to be known as Shark Alley. Here, apparently because the water is slightly shallower (not as much runway room), the sharks don’t usually breach; instead, they seem to swarm.