How does an 18-foot-long, 2,000-pound carcass just disappear?

That question has puzzled some divers and photographers who regularly plunge into the waters off San Diego.

It started earlier this spring when Doug Bonhaus took advantage of some calm weather to scuba dive in Scripps Canyon. As he descended, a hulking mass took shape below him.

There, at an exceptionally shallow 115 feet, lay the body of a baby gray whale.

Whale falls are usually not seen by human divers. Typically, they are discovered by remotely operated vehicles at depths exceeding 3,000 feet.

Local marine biologists had a guess as to the gray whale calf’s origins. An animal that matched what was found on the seafloor had been spotted swimming near La Jolla Shores, desperately searching for its mother. During its final hours, it was seen approaching boats, as though asking for help that wasn’t coming.

Because it was the first time in memory that a fall was so accessible to people, other divers quickly made their way to the site. Among them was Jules Jacobs, an underwater photojournalist who has written for The New York Times about his explorations.

At that point in late January, the carcass’s resting place was a trough in the canyon that required pinpoint precision to reach. So Mr. Jacobs steeled himself for a dangerous and mentally taxing dive.