Scientists are monitoring an iceberg roughly six times the size of Manhattan - one of the largest now in existence - that broke off from an Antarctic glacier and is heading into the open ocean. NASA glaciologist Kelly Brunt said on Wednesday the iceberg covers about 255 square miles (660 square km) and is up to a third of a mile (500 meters) thick. Known as B31, the iceberg separated from Antarctica's Pine Island Glacier last November, Brunt added. "It's one that's large enough that it warrants monitoring," Brunt said in a telephone interview, noting that U.S.