This past fall, two hundred people gathered at The Explorer's Club in New York City. The building was once a clubhouse for famed naturalists and explorers. Now it's an archive of ephemera and rarities from pioneering expeditions around the globe. But this latest gathering was held to celebrate the first biological census of its kind –an effort to count all of the squirrels in New York City's Central Park. Squirrels were purposefully introduced into our cities in the 1800s, and when their population exploded, we lost track of how many there are.