Chesapeake Bay researchers have said they have counted more than 1,000 bottlenose dolphins congregating near the Potomac River, according to The Washington Post.

The marine mammals have been seen in large numbers near the end of the river, where the water is cleaner and it widens to flow into the bay, the paper reports.

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According to the news outlet, the researchers think that dolphins not only are visiting the Potomac but are thriving there, with some from Georgetown University saying they witnessed a dolphin birth, something that is incredibly rare to see outside of captivity.

The associate director of the Potomac-Chesapeake Dolphin Project Ann-Marie Jacoby told the Post “There are dolphins here, and there’s breeding and birthing going on, and this is connected to D.C. — such a populated, urban area.”

According to the Post, newspaper clippings from the 19th century show that dolphins, then referred to as porpoises, were spotted in the Potomac.

Researchers told the Post that an outbreak of cetacean morbillivirus killed thousands of dolphins along the Mid-Atlantic from 2013 to 2015 convinced them they needed to do studies on the area.

A study to further observe the dolphins that make their home so close to D.C. started in 2015 and now runs from May through October every year.

During the study, dolphins have been spotted near the Potomac River Bridge, which is only 50 miles from Washington.