“When we think of modern-day international border disputes, we imagine places like Kashmir, Jerusalem, Crimea, or the U.S.-Mexico border,” filmmaker Brian Gersten told The Atlantic. “So when I heard about Machias Seal Island, it was just so unexpected.”

Gersten is referring to the last remaining border conflict between the U.S. and Canada. Nicknamed “the grey zone,” the 20-acre island sits in the Gulf of Maine. Despite the fact that its sovereignty—and that of the 277 square miles of ocean that surrounds it—is contested, the Canadian Coast Guard staffs a lighthouse on the island. Meanwhile, many fishermen from both the U.S. and Canada regard it as a precious hunting ground.

“We have this 250-year-old border dispute with one of our closest allies—and it concerns a rock in the middle of the ocean where a bunch of puffins live. That's some weird, wild stuff,” Gersten said.

Gersten’s short documentary, The Grey Zone, surveys the history of the skirmish and investigates its current implications. It’s a story as old as the Revolutionary War, involving mythological figures and lobsters. (When the U.S. and Canada submitted other border claims to the International Court of Justice at the Hague in 1984, Machias Seal Island was conspicuously absent.)

In the film, experts on the matter muse on the absurdity of what they argue are lines drawn in the land. “Nation-states are imagined communities,” says Setha M. Low, Professor of Anthropology at The Graduate Center, CUNY.

“The way people talk about borders these days is like they were handed down from God,” says Stephen Kelly, a scholar at Duke University and a former diplomat, “but most people don’t realize how arbitrary and erroneous they are.”