A century ago, dozens of shipyards across the United States constructed a fleet of wooden steamships to aid the fight against Germany during World War I. Today, ospreys nest on the boats, and bats breed in the hull. More than 100 of these historic vessels survive, serving as a half-submerged home for fish, beavers, waterfowl, and vegetation along a stretch of the Potomac River next to Mallows Bay, Maryland.

On Monday, the National Oceanic and Atmospheric Administration designated the 18-square-mile area a national marine sanctuary—the first in nearly two decades. The Mallows Bay-Potomac River National Marine Sanctuary holds not only remains of the “ghost fleet” of WWI vessels, but also Civil War-era shipwrecks, and Native American archaeological sites dating back