The Mariana Trench or Marianas Trench is the deepest part of the world's oceans.It is located in the western Pacific Ocean, to the east of the Mariana Islands.he Mariana Trench is part of the Izu-Bonin-Mariana subduction system that forms the boundary between two tectonic plates. In this system, the western edge of one plate, the Pacific Plate, is subducted (i.e., thrust) beneath the smaller Mariana Plate that lies to the west.