Simplify the following text for a Grade 6 native English-speaking person.
Original: The Mariana Islands are the southern part of a submerged mountain range that extends 1,565 miles ( 2,519 km ) from Guam to near Japan .
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 .