The boot camps, which run for three days and have been held since 2003, are aimed at "educating" civilians about national security in a country that shares a heavily-armed border with North Korea, which in November shelled a South Korean island near their shared sea border, killing several civilians. About 250 people, including some high school students, took part in the boot camp at Bucheon, just west of Seoul, one of six run around the country. Instead of staying warm at home during their winter break, participants wearing camouflage dragged parachutes, underwent training in a tear-gas filled hut and took part in "flying fox" exercises from a wooden tower. The cold was unforgiving, with temperatures hitting -10 C (14.00F) in Seoul and surrounding areas. "It was very difficult from the beginning, my muscles ached and it hurt a lot," said 18-year-old Kim Myung-jin. "But I trusted my trainers under the intense situations and sympathized with much of the soldiers' hard work."