South Korean Marines exit an amphibious assault vehicle during a joint landing operation by US and South Korean Marines in Pohang, 270 kms southeast of Seoul, on March 31, 2014.  Meanwhile, the two Koreas traded hundreds of rounds of live artillery fire across their disputed maritime border, forcing South Korean islanders to take shelter a day after the North drove up tensions by threatening a new nuclear test.     AFP PHOTO / JUNG YEON-JE        (Photo credit should read JUNG YEON-JE/AFP/Getty Images)