Small gray porpoises have recently reappeared in San Francisco Bay after leaving the waters more than 60 years ago—and researchers are trying to understand why.

The marine mammals, called harbor porpoises, fled the Northern California waters during World War II. The area was a major shipping route during the war. Porpoises have sensitive hearing and need lots of room to leap and hunt, and the shipping lanes made the bay too noisy and dangerous for the animals.

When the war ended, the area became home to many factories and businesses, some of which polluted the waters. The contaminated waters likely drove away the porpoises completely.

But thanks to the Clean Water Act of 1972—a federal law aimed at improving the quality of surface waters—industries could no longer simply discharge their waste into waters and pollute waterways. Instead, they had to adhere to standards and limits on what could be put into lakes, bays, rivers, and streams.

Since then, the water quality of the bay has improved. Scientists believe this could be why the porpoises have returned.

So far, researchers have identified 250 porpoises in the bay.

“It’s one of those very few good-news environmental stories. And it’s in our backyard. It gives one hope,” Jonathan Stern, a San Francisco State University whale researcher, told NPR.

Stern and his colleagues are studying the porpoises more closely to learn about their life and habits.