Original: The Mariana Islands ( also the Marianas ; up to the early 20th century sometimes called Ladrones Islands , from Spanish Islas de los Ladrones meaning '' Islands of Thieves '' ) are an arc-shaped archipelago made up by the summits of 15 volcanic mountains in the north-western Pacific Ocean between the 12th and 21st parallels north and along the 145th meridian east .
Simplified: They are the southern part of a submerged mountain range that goes on for 1,565 miles ( 2,519 km ) from Guam to near Japan .