ARCHIVE: 25 Things You Should Know About the Deep Sea: #1 The Definition of Deep Sea

The deep sea represents those marine environments that occur beyond the continental shelf. The average depth at which this occurs is approximately 200m, so typically we define the as those environments greater than 200 meters and extending to approximately 10,000 m (the depth of Mariana’s Trench, the deepest point in the ocean).

Although the defining feature is not depth per se but rather the transition from the continental shelf to the continental slope, referred to as the shelf break. The term deep sea can be used to characterize both deep pelagic habitats, the water column greater than 200m, and deep benthic habitats, the seafloor greater than 200m. The slope is the region beyond the shelf where the seafloor rapidly increases in depth (~200-2000m although these estimates rely solely on the topography of the region). From 2000-4000m (again approximate) the seafloor’s depth increases more gradually and this section is denoted as the continental rise. Both the slope and rise are referred to as the bathyal region. Eventually at great depths, very little change occurs in depth and these large extents of the bottom are referred to as the abyss or abyssal plains. Of course all of these areas can be perforated by trenches with depths up to 10,000m.

Dr. M (1730 Posts)

Craig McClain is the Assistant Director of Science for the National Evolutionary Synthesis Center, a National Science Foundation supported initiative. He has conducted deep-sea research for 20 years and published over 50 papers in the area. He has participated in and led dozens of oceanographic expeditions taken him to the Antarctic and the most remote regions of the Pacific and Atlantic. Craig’s research focuses on how energy drives the biology of marine invertebrates from individuals to ecosystems, specifically, seeking to uncover how organisms are adapted to different levels of carbon availability, i.e. food, and how this determines the kinds and number of species in different parts of the oceans. Craig’s research has been featured on National Public Radio, Discovery Channel, Fox News, National Geographic and ABC News. In addition to his scientific research, Craig also advocates the need for scientists to connect with the public and is the founder and chief editor of the acclaimed Deep-Sea News (, a popular ocean-themed blog that has won numerous awards. His writing has been featured in Cosmos, Science Illustrated, American Scientist, Wired, Mental Floss, and the Open Lab: The Best Science Writing on the Web. His forthcoming book, Science of the South (, connects cultural icons of South such as pecan pie with the science behind them.