the regions beneath the surface of the oceans and seas.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
The regions beneath the ocean's surface, especially when considered as an area to be studied, as in marine mammal research or sonar mapping of the ocean floor.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.