When a whale dies and its corpse falls to the bottom of the ocean, entire ecosystems rapidly develop around eating every part of it due to how scarce resources are in the deep ocean. This phenomenon is called a "whale fall" and it's a major source of energy for deep ocean ecosystems.
Locally sourcing boulders over the Marianas Trench is going to be such a pain. I'm pretty sure the environmental benefits will outweigh importing some nice basalt from Hawaii before we leave out.