From
Wikipedia:
Located in an area rich with American history, scholars believe the span crosses within 100 yards of the site, marked in the water off the bridge by a stars and striped painted buoy, where Francis Scott Key witnessed the bombardment of Fort McHenry on the evening of September 12, 1814. That battle inspired Key to write the words of the Star Spangled Banner. Located just southeast of the bridge are the ruins of Fort Carroll.
Ladies and Gentlemen I present to you, the Francis Scott Key buoy:

Francis Scott Key buoy