Simplify the following text for a Grade 13 native English-speaking person.
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: The Mariana Islands ( also the Marianas ; up to the early 20th century sometimes called Islas de los Ladrones meaning `` Islands of Thieves '' ) are a group of islands made up by the summits of 15 volcanic mountains in the western Pacific Ocean .