 FACT: Dr. Zimsky states that he chose the Marianas Trench as the  launch location because the crust is thinner there. The Marianas  Trench is a subduction zone, where the Pacific plate is being pushed  under the Philippine plate. Large amounts of sediments are built up  at subduction zones as one plate scrapes the top off the other  plate, so the oceanic crust is actually a good bit thicker at the  trench. A better location would have been in the middle of a plate,  such as near Hawaii.