Harold Warner, a volunteer at the Hawthorne Ordnance Museum, holds one of the more curious objects: a submarine knocker, Wednesday, Oct. 23, 2013. Sailors would drop these heavy magnets in hopes they would stick on enemy submarines. Hanging below the magnet was a piece of metal that acts like a door knocker, which if stuck to a submarine would continue to ping against the hull. That would give sailors above a way to locate the enemy. Below it is the tail of a depth charge.