A sight that hasn't been seen since before World War II is making a welcome comeback beneath the most famous bridge in Northern California. Harbor porpoises are showing up in growing numbers under the Golden Gate Bridge. Six decades ago, they were driven away in part by wartime activities and poor water quality; now, researchers are trying to understand why they're returning.

The walkway across the Golden Gate Bridge is almost always packed with people taking photos. But Bill Keener isn't here for snapshots of the stunning views. He's aiming his massive telephoto lens at a dark shape in the water 200 feet below.

"There's a porpoise right there, coming very, very close," he says. "Here's a mother and calf coming straight at us." Keener is with Golden Gate Cetacean Research, a nonprofit group focused on studying local porpoises, whales and dolphins.

Harbor porpoises have dark gray backs, and they're about 5 feet long — smaller than most of their dolphin relatives. Keener spots one turned on its side and spinning.

The porpoises, feeding in the middle of a busy shipping lane, spin as they go after schools of herring and anchovies. Seeing this behavior is huge for Keener because harbor porpoises are notoriously shy in the open ocean. But the fact that they're here at all is what's most remarkable.

Keener and his colleagues have identified 250 porpoises with their photos by looking for unique scars on the animals. When the team first started working on the bridge, the patrol officers took notice.

"We're staring down at the water for hours," Keener says. "They start getting worried about us. But they know us now; they know what we're doing."

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