Task: Question Generation
Input (English): During most of the war, the UN navies patrolled the west and east coasts of North Korea, sinking supply and ammunition ships and denying the North Koreans the ability to resupply from the sea. Aside from very occasional gunfire from North Korean shore batteries, the main threat to United States and UN navy ships was from magnetic mines. During the war, five U.S. Navy ships were lost to mines: two minesweepers, two minesweeper escorts, and one ocean tug. Mines and gunfire from North Korean coastal artillery damaged another 87 U.S. warships, resulting in slight to moderate damage.
Output (English): What weapon posed a significant threat to US navy ships?