On a Wednesday afternoon in Brooklyn, a group of students, staff, and volunteers with the Billion Oyster Project are about to get their hands dirty. Led by Field Coordinator Tatiana Castro, they'll be wading into a lagoon to count and measure oysters.
"The reason why we do spring monitorings is because we want to be able to measure how many oysters made it through overwintering," Castro explained.
That there are even oysters to monitor in New York Harbor is remarkable. Over 100 years ago, they became functionally extinct in New York City as a result of overfishing and pollution that rendered them inedible. But Billion Oyster Project is working to change that by restoring one billion live oysters to the harbor, and in so doing creating more resilient waterways and communities.
"An adult oyster can filter up to 50 gallons of water a day, just one adult oyster … and so the reason why we chose a billion is because that's how many oysters it would take for the oysters to filter the entire harbor," said Castro, her gloved hands covered in harbor muck as she took inventory of the oysters. 
Aside from filtering water, oysters create reefs that help reduce the intensity of waves, which is vital for New York's vulnerable shorelines. They also contribute to biodiversity by creating three-dimensional habitats for other marine life. 
Since its founding in 2014, Billion Oyster Project has organized more than 11,000 volunteers, more than 8,000 students, and 200 New York City schools to assist in its efforts. And they've made progress, restoring 75 million oysters to New York Harbor across 16 acres of habitat, said Katie Mosher, the project's senior director of programs.
"We've done this by building reef habitat that's been partially created by volunteers," Mosher explained.
The organization relies on volunteers and places a strong emphasis on educational programming designed to teach about climate change and reconnect New Yorkers with waterways that are theirs to enjoy.
"In New York City, people are not very connected to the water. And so, for us, programming around oyster reef restoration is one way to increase every community's understanding of the potential for harm that can be caused by sea-level rise," Mosher said. 
"We can come to the water, we can connect with nature, we can connect with marine organisms, and they are out here they're alive," she later added.