intensified maintenance. all preparing for what the u.s. says is a north korea contingency but officials can't say what that contingency might be. >> they see that possibility of north korea were to implode as their biggest geopolitical worry. >> reporter: as tensions rise across the region, the "uss carl vinson" strike group is expected to arrive within days. it's planes will conduct visible flight operations. u.s. submarines will be able to remain covert under sea conducting surveillance of north korean communications if ordered. a joint u.s. south korea military exercise named max thunder now underway. the second largest air exercise held on the peninsula. all aimed at insuring the security of south korea and the 28,000 u.s. troops there. no let up in the militaristic