Captain Francis D. Lee invented the frame torpedo-mine to help defend his native Charleston, South Carolina. The captain filled cast-iron shells with gunposder, then mounted them on heavy timbers, which he secured to the bottom of the harbor. Lying three feet under water, these would detonate when a ship struck the percussion igniters in the torpedo's casing. Charleston's defenders placed these mines in the Ashley River in March 1863 and between Fort Sumter and Sullivan Island in July 1863. After the fall of Charleston, Union engineers removed all of the torpedoes from the harbor. Ironically, the only ship to be sunk by this weapon was the Confederate transport Marion, which accidentally struck one on the night of April 6, 1863.