South Korea's military fired warning shots and broadcast messages with loudspeakers after ten or so North Korean soldiers crossed into the DMZ at 5PM. After the warnings, the Norks returned to the North. Some of the North Korean troops were armed and wearing bulletproof vests. The motive for Tuesday's border crossing by the North Korean soldiers wasn't immediately clear. The 155-mile-long, 2.5-mile-wide DMZ is the world's most heavily armed border. An estimated two million mines are peppered inside and near the border, which is also guarded by barbed wire fences, tank traps and combat troops on both sides. Below, ROK soldiers patrol along the barbed-wire fence in Paju, South Korea.