Many visitors flock to coastal Maine for lobster rolls, kayaking, and potato doughnuts. But military history buffs will find lots to love, too—if they’ve brought waterproof boots and a flashlight.

During World War II, Peaks Island, a popular summer destination in Portland, Maine, was home to a big, bustling military installation. Because the U.S. military worried that the East Coast could be vulnerable to attacks from enemy submarines or ships, the government seized nearly 200 acres on the island via eminent domain and built 58 buildings, where some 800 soldiers lived and worked.

The largest of these was Battery Steele—among the biggest gun batteries ever built in America. The concrete building is several hundred feet long, but easy to miss even if you’re right in front of it: From the get-go, it was deliberately concealed with soil and plants. Inside, battleship guns pointed out at the water. They were more than 60 feet long with barrels measuring 16 inches in diameter, and could fire 2,000-pound projectiles roughly 26 miles. Local lore holds that test shots shattered windows across the island. Though the guns were never fired in anger, islanders lived in the shadow of that threat.

In a way, they still do, even though the guns were scrapped after the war. In the video above, Atlas Obscura visits the Fifth Maine Regiment Museum to learn about the island’s wartime history—from dark, damp, graffiti-covered Battery Steele itself to long-abandoned observation towers deep in the forest.

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