Human New Yorkers no longer have any use for Swinburne Island. The nonhumans, however, are taking advantage of the little sliver of open real estate. Increasing numbers of harbor seals have moved in and been happily—or at least loudly—calling it home.

There are nearly 100 harbor seals on Swinburne Island, which easily makes it their most densely populated habitat in New York City. It’s pretty secluded, too, as far as New York City goes. There are no beach walkers to disrupt them, no car horns, and no barking dogs.

Swinburne and nearby Hoffman Islands are artificial islands, originally built with landfill in the 1870s to house quarantine hospitals for newly arrived immigrants. Anyone thought to be too sick to pass through Ellis Island were sent there to get healthy.

With advancements in the field of infectious disease research and lower immigration numbers following World War I, quarantine hospitals became less necessary. In the 1930s, the Merchant Marines used Swinburne island for training. After that, it mostly went unused. Finally, in 1972, the island became part of the Gateway National Recreation Area, where it is currently managed by the National Park Service.

Initially, the abandoned island was home to cormorants and seagulls. Then the seals arrived. According to Paul Sieswerda, former curator of the New York Aquarium, the seals were actually returning to the area. He believes they came back sometime around 2008. The seals owe a lot of their resurgence to The Marine Mammal Protection Act of 1972, which protects seals and other marine mammals.