Guam, a territory of the United States, is the largest and southernmost of the Mariana Islands in the north-western Pacific Ocean. It is 30 mi long and about 8 mi wide. The northern portion of this tropical island is a plateau of coral formation; the southern portion is hilly and of volcanic origin. Good anchorage is in Apra Harbour, on the western coast and the capital, Agana, is to the north. Guam is one of the principal US defense fortifications in the western Pacific Ocean and is the site of extensive naval, army, and air force installations.