Archaeologists searching Fort McHenry for underground drainage conduits have stumbled upon a long trench presumably used by infantrymen the night the British attacked Baltimore in 1814.

The bombardment of Fort McHenry inspired Francis Scott Key to write the poem "The Star-Spangled Banner," the national anthem.The trench, originally about 3 feet deep, is about 7 feet below today's surface level and is more than 100 feet long. It contained a pewter infantryman's button the size of a nickel and a 10-pound fragment of a British cannon ball.

The trench was dug around the fort's western flank, the side farthest from the harbor and the British fleet. Historians believe the 600 infantrymen stationed at Fort McHenry could have been ordered into the trench as a safety measure.

If the British had landed successfully in the attack on the night of Sept. 13, 1814, the infantrymen would have been called upon to defend the city.