The Mariana Islands (also the Marianas) are an arc-shaped archipelago made up by the summits of fifteen volcanic mountains in the north-western Pacific Ocean between the 12th and 21st parallels north and along the 145th meridian east. They are south of Japan and north of New Guinea, and form the eastern limit of the Philippine Sea. The islands were named after Spanish Queen Mariana of Austria in the 17th century, when Spain started the colonization of the archipelago. They form the northern part of the western Pacific subregion of Micronesia, and are composed of two U.S.