The Undersea is a Discrete Operational Domain

By: Will Spears
Reading Time: 7 Minutes

Today there are five doctrinally recognized operational domains: land, maritime, air, space, and cyberspace. The DOD Dictionary, citing Joint Pub 3-32, defines the maritime domain as the “oceans, seas, bays, estuaries, islands, coastal areas, and the airspace above these, including the littorals.” While it provides definitions for all the recognized domains, it leaves the term domain conspicuously undefined. Dr. Jeffrey Reilly defines domain as a “critical macro maneuver space whose access or control is vital to the freedom of action and superiority required by the mission.” This short commentary will propose that the undersea, those areas of the oceans, seas, bays, estuaries and littorals extending from the surface to the seafloor, is itself a discrete operational domain in accordance with any reasonable definition of the term.

Just as multidomain operations are nothing new, identifying the undersea as a specific domain onto itself is also nothing new. For example, according to its commander, the mission of the U.S. Submarine Force is “to execute the mission of the U.S. Navy in and from the undersea domain.” It isn’t likely that that the Submarine Force is intentionally at odds with joint doctrine in identifying the undersea as a separate domain, but rather that it hasn’t yet become important for various arms of the U.S. military to get on the same page as to what a domain is. As multidomain operations continue to transition from Pentagon buzzwords to modus operandi, minor glitches like the above will be quietly ironed out.

It matters because conceptual accuracy matters. The models we use to represent complex ideas turn into mental shortcuts that, for good or ill, end up shaping our way of seeing things. These views shape discussion and debate, which in turn shape plans and policy. To lump the undersea together with the ocean’s surface and the air above it is to over-generalize to the point of obscurity, pushing the undersea, its opportunities and its vulnerabilities into the periphery of consciousness. Our undersea forces are comfortable in obscurity, but our joint commanders require plans and policy that are developed with full appreciation of all opportunities and vulnerabilities at play.

Why the undersea is different

Skeptics may argue that the maritime domain is an unnecessary hair to split. We don’t, after all, sub-divide land into mountain, jungle, desert, or urban domains. That’s because these are types of terrain, and while they require different training and doctrine to negotiate, they are not fundamentally different spaces of maneuver. The ocean also has terrain— ice, littorals, open ocean, archipelagic waters, or high-traffic straits for example—each requiring its own training and doctrine, each presenting its own challenges. These challenges manifest very differently above or below the ocean’s surface, though. Polar ice, for example, affords a haven for undersea forces while denying access to surface ships. Those same surface ships can maneuver unimpeded through shallow waters that submarines may find challenging or inaccessible.

Even in open ocean, the shape and composition of the seafloor matters to undersea forces. Submarines sense their environment through vibrations, which travel much faster in water than in air. Sound travels any direction but straight in water, reflecting or refracting from the surface, seafloor, or thermoclines, and interacting with ocean weather like fronts, eddies or ocean currents. A sloping or mountainous seafloor will have different characteristics than a flat seafloor, and clay will reflect sound differently than rock or sand. These effects converge to produce a variety of phenomena such as sound channels, surface ducting, or convergence zones, all of which are important to understanding the undersea domain.


Radar is not a factor underwater. Visible light barely penetrates the ocean’s surface. Undersea forces minimize communication by necessity, naturally operating with a mission command philosophy that is deeply ingrained in submarine culture. These factors converge to make undersea forces the least vulnerable of any military assets to effects in the electromagnetic spectrum, as well as the least likely to be crippled by an attack on space assets. Unfortunately, these factors also make it hard to communicate with submarines, making coordinated submarine operations a cumbersome task that is often frustrating for addicts of nonstop communications.

domain_stackMuch like the space domain is closely linked with the air, the undersea domain is closely linked with the surface. Undersea forces must transit the surface domain to access the undersea and must return to the surface for repair and replenishment. Unmanned undersea vehicles are frequently deployed from surface ships, and sea mines are undersea weapons with effects in both domains. Like space, the undersea domain is critical to the information-age economy, with substantial energy assets and an elaborate web of cables supporting international communications. Unlike space, the undersea is home to vast arsenals of very lethal capability, including the sea-based deterrents fielded by every major nuclear weapons power.

Any future contest for command of the sea will hinge upon what happens in the undersea. Neither party may claim to have achieved sea control while either the surface or the undersea remain contested. Likewise, neither party is denied the sea while they can still operate on or under the surface with acceptable levels of risk.  For example, continental powers may aspire to deny access by holding naval surface forces at risk with inexpensive land-based missiles. Without a complementary ability to hold undersea forces at risk, access cannot be denied, only contested.


In a rapidly evolving high-end conflict, control of any domain will be constrained in time as well as geography. Operating inside of highly dynamic “windows of superiority” requires rapid-fire decision-making informed by an organic understanding of the opportunities and vulnerabilities presented in every domain. This understanding is better served by a conceptual model that examines the surface and undersea as closely linked but discrete operational domains.

A Problem of Concept

A separate undersea domain does not mesh well with the “every service operates in every domain” narrative that seems to be critical in conceptually separating joint from multidomain. No service other than the Navy has the equipment or expertise to produce effects in or from the undersea domain, nor should they aspire to. This is not a problem of practicality, but one of concept, in that acknowledging a separate undersea domain will require refinement of language in other aspects of multidomain thought. It is not as simple as splitting the maritime domain in two.

Put simply, the five-domain model of land, maritime, air, space, and cyberspace is not well thought out. Rather, it is a baby step toward multidomain thinking, derived from little more than attaching space and cyberspace to the traditional responsibilities of the Army, Navy, and Air Force. The doctrinal definition of maritime as the “oceans, seas, bays, estuaries, islands, coastal areas, and the airspace above these, including the littorals” could be more simply stated as “things the Navy is responsible for.” This is a definition that emerged with an established service in mind and is not sufficiently precise to support future evolution of multidomain thought.

While this commentary is intended to provoke debate rather than to establish any sort of precedents, I offer the following as a point of departure for missing definitions. Instead of an all-encompassing maritime domain, there is a surface domain and an undersea domain. The undersea is defined as those areas of the oceans, seas, bays, estuaries and littorals extending from the surface to the seafloor. The surface is defined as those areas of oceans, seas, bays, estuaries and littorals that are navigable by marine vessels lacking the capability to submerge (i.e. polar ice regions are excluded from the surface domain). Islands and coasts are land, and riverine forces belong to the land domain as well. The air is the air, regardless of whether land or sea is under it.

Conceptual Accuracy Matters

To operators, military theorists debating nebulous concepts can sound a lot like philosophers babbling over semantics, ultimately producing flow charts and Venn diagrams with zero utility to the warfighter. That’s not what the multidomain imperative, or this commentary, is about. Regardless of what conceptual banner the transition falls under, seams in between operational domains are going to become increasingly transparent as a matter of necessity and natural evolution. Clarity of thought and language is a necessary and heretofore untaken first step to ensuring this transition occurs in accordance with a plan. Given the stakes, it isn’t asking too much to expect that it’s done right.

LCDR Will Spears is a U.S. Navy submariner and a student in the Multidomain Operational Strategist concentration at the Air Command and Staff College. He has served aboard multiple attack submarines in the Western Pacific area of responsibility. Email:

The views expressed are those of the author and do not necessarily reflect the official policy or position of the Department of the Navy, the Submarine Force or any organization of the US government.

Iceberg feature image from Underwater sound profile image from unclassified publication Fleet Oceanographic and Acoustic Reference Manual. Other images are original.


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