Journalist Susan Casey was in her living room when she first glimpsed this strange place and its resident sharks, their dark fins swirling around a tiny boat in a documentary. These great whites were the alphas among alphas, the narrator said, some of them topping 18 feet in length, and each fall they congregated here off the northern California coast. That so many of these magnificent and elusive animals lived in the 415 area code, crisscrossing each other under the surface like jets stacked in a holding pattern, seemed stunningly improbable and irresistible. Casey knew she had to see them for herself.

Within a matter of months she was in a 17-foot Boston Whaler, being hoisted up a cliff to face onto the barren surface of Southeast Farallon Island, part of the group known to 19th-century sailors as the "Devil's Teeth". There she joined the two biologists who study the sharks, bunking down in the island's one habitable building, a haunted, 120-year-old house spackled with lichen and gull guano. Less than 48 hours later she had her first encounter with the famous, terrifying jaws and was instantly hooked. Curiosity yielded to obsession, and when the opportunity arose to return for a longer stay she jumped at it. But as Casey readied herself for shark season, she had no way of preparing for what she would find among the dangerous, forgotten islands.