NEDU: Rising to the Challenge

Navy divers, Explosive Ordnance Disposal, and Special Forces perform difficult tasks in dangerous underwater environments. Since 1927, the Navy Experimental Diving Unit (NEDU) has safeguarded divers and expanded diving capabilities. NEDU’s ongoing mission is to research, develop, test, and evaluate diving equipment and procedures. Put more simply, NEDU’s job is problem-solving: NEDU personnel use their expertise and experience to find solutions for the many challenges of working underwater. Over the last nine decades, its work has significantly extended the depth, duration, and safety of Navy diving.

Learn more about NEDU and its most significant accomplishments by using the tabs below:

Why an Experimental Diving Unit?

Two submarine sinkings in the 1920s revealed significant shortcomings in the Navy’s rescue and recovery abilities. The Navy formed the Experimental Diving Unit in 1927 to address these deficiencies. NEDU’s early work focused on the Navy’s immediate needs: improving submarine rescue and escape capabilities, developing breathing mixtures that extended diving depths, and establishing decompression tables and treatment practices. In the 1960s and 1970s, with the fundamentals in place, NEDU shifted its attention to creating saturation diving procedures and to evaluating gear for divers, EOD personnel, and Special Forces.


What Exactly Does NEDU Do?

NEDU’s team of scientists, engineers, and divers ensures diver safety and expands diving capabilities in two key ways:

Testing and Certifying Diving Equipment

Most of the Navy’s diving gear is developed by commercial contractors for time and cost efficiency. NEDU evaluates this equipment to be sure it meets the Navy’s rigorous safety and performance standards before being placed in service.

Developing Diving Procedures

Military diving is complicated, hazardous work. Divers must adhere to exact rules and procedures or risk illness or death. NEDU develops the precise procedures, like decompression tables or treatment steps, that divers follow to remain safe.

Submarine Rescue and Escape

The Challenge: The Navy lacked means of saving survivors of submarine accidents.

The Solution: The Navy founded NEDU to develop rescue and escape methods for trapped submariners. NEDU personnel designed and tested rescue and escape equipment and techniques. In the early 1970s, Submarine Development Group One assumed primary responsibility for submarine rescue after the Navy adopted new rescue vehicles called DSRVs. NEDU continued to support the rescue and escape program through evaluation of equipment and procedures.


Early Equipment

In 1928, NEDU tested a wearable escape device called a Momsen lung after its inventor, LT Charles Momsen. The Momsen lung provided the first successful means of submarine escape. It became the Navy’s primary escape method for over 25 years. A few years later, LCDR Allan McCann, working with Momsen, completed the design for a submarine rescue chamber. The chamber could be lowered to a downed submarine to raise survivors to the surface. NEDU evaluated and certified the rescue chamber in the early 1930s.


Buoyant Ascent Technique

In 1956, the Navy introduced a new approach to submarine escape, the buoyant ascent technique. NEDU developed and carried out the test program for the technique, which called for a submariner to continuously exhale while ascending to the surface. NEDU found this new approach to be safer than the Momsen lung, which could cause injury or death if used incorrectly.


JIM Suit

Commercial inventors designed the JIM suit, the first modern atmospheric diving suit (ADS), in the early 1970s. ADS maintain surface pressure internally and eliminate the need for decompression. NEDU evaluated the JIM suit in 1975 and concluded ADS would be valuable tools for submarine rescue missions. As a result, the Navy initiated development and use of ADS. ADS allow divers to investigate downed submarines and clear their hatches for rescue equipment.



In more modern times, NEDU helped test protective suits for submariners escaping a disabled submarine. The submarine escape immersion equipment (SEIE) suit improves its wearer’s survivability by providing thermal protection and floatation while awaiting rescue.

Helium-Oxygen Breathing Mixture

The Challenge: Breathing compressed air below depths of 165 feet could cause nitrogen narcosis, impairing diver judgment and awareness, which endangered the diver and the diving operation.

The Solution: In the 1930s, NEDU developed and perfected a new breathing gas mixture using helium. Breathing helium-oxygen made deep-sea diving safer and let divers dive deeper. Today using helium is standard Navy practice for surface-supplied dives to 190 feet or greater.


USS Squalus Salvage

In May 1939, submarine USS Squalus (SS-192) sank in 243 feet of water. The salvage operation provided the first real test of NEDU’s helium-oxygen breathing mixture. After divers experienced disorienting nitrogen narcosis while breathing air, they were switched to helium. Breathing helium eliminated the effects of nitrogen narcosis and allowed the divers to work longer and more efficiently.


Mark V Mixed Gas Rig

Helium was not only expensive, it also increased a diver’s buoyancy. Adopting helium as a breathing gas required the Navy to develop a helium version of its surface-supplied diving rig, the Mark V. The helmet was modified to recycle breathing gas by scrubbing (removing) carbon dioxide, reducing helium costs. To compensate for the positive buoyancy helium introduced, the rig’s weight was increased by 100 pounds.

Decompression Procedures

The Challenge: As a diver descends underwater, water pressure forces extra nitrogen into the diver’s body tissues. Divers must decompress (safely expel the nitrogen) when they return to the surface. Inadequate decompression can cause decompression sickness, a dangerous condition where gas bubbles form in body tissues.

The Solution: NEDU’s medical personnel began studying decompression sickness in the 1930s. They created procedures that helped divers decompress more correctly and developed methods for treating decompression sickness.


Decompression Tables

Decompression tables, first introduced in 1908, tell divers how to safely decompress after a dive. Divers use different tables depending on the dive’s depth and duration, the type of breathing mixture used, and the availability of a decompression chamber.

Beginning in the 1930s, NEDU developed key improvements to existing decompression tables when diving on air. Over time, NEDU developed additional refinements and new types of decompression tables for different breathing mediums and different diving equipment. Many have become the standard tables used worldwide.


Treating Decompression Sickness

NEDU personnel also devised treatment procedures for divers suffering from decompression sickness or gas embolism (bubbles in the bloodstream). Early treatment relied on re-pressurizing affected divers to deep depths while breathing compressed air. Improved treatment tables, introduced in the 1960s, yielded more effective results by administering oxygen therapeutically at shallower depths.

Saturation Diving

The Challenge: Surface-supplied diving limited the length of time divers could work underwater, and required lengthy decompression periods for short bottom work times.

The Solution: Dr. George Bond, a Navy scientist, introduced the concept of saturation diving the 1950s. It allowed divers to live and work underwater for days or weeks at time before making a single, comparatively short decompression period. NEDU provided practical support for Dr. Bond’s hypothesis by developing special saturation diving decompression tables. More recently, it tested and certified the Navy’s newest saturation diving system.


The Sealab Projects

In the 1960s, Dr. Bond staged a trio of saturation diving experiments that let divers work and live in undersea habitats called Sealabs. NEDU provided training and support to the Sealab projects, most significantly by developing and testing the decompression schedules divers used after days or weeks underwater. Because it had to address the effects of both saturation diving and the use of helium as a breathing mixture, these procedures were more complicated to develop.

To prepare for Sealab III, NEDU divers performed 28 helium-oxygen saturation dives to depths of 825 feet over a period of three years. NEDU medical officers used the findings from these dives to establish safe decompression procedures for the Sealab III aquanauts.


Saturation Diving Decompression Tables

In addition to the Sealab decompression tables, NEDU personnel developed tables for other saturation diving applications. In the late 1960s, the Navy introduced the Mark 1 Deep Diving System, its first saturation diving support system. NEDU tested and established the decompression procedures for divers to use after working from the Mark 1.

In the 1970s, NEDU pioneered special decompression tables that allowed saturation divers to change depth as they worked. Before the new procedures, if saturation divers changed their working depth their decompression procedures would be skewed, risking incorrect or extended decompressions.


The Monitor Salvage Expeditions

In the late 1990s the Navy undertook a project to raise portions of the sunken Civil War ironclad USS Monitor as a training exercise for Navy divers. During the 2001 and 2002 operations, the Navy relied heavily on saturation diving to accomplish most of the underwater work. NEDU supplied the saturation divers and provided saturation diving guidance and expertise to the project.


Saturation Fly-Away Diving System

The effective use of saturation diving during the USS Monitor expeditions revived the Navy’s interest in maintaining its own saturation diving system. A decade after retiring its last deep diving system, the Navy initiated development of a new, highly mobile Saturation Fly-Away Diving System (SAT FADS). NEDU handled all manned testing and certification of the new system.

Equipment Testing

The Challenge: The Navy needs to continually improve diving capabilities while maintaining diver safety.

The Solution: In modern times, NEDU’s most important task is evaluating diving gear diving gear for use by Navy divers, Explosive Ordnance Disposal (EOD), and Special Forces. NEDU’s exhaustive testing ranges from ensuring equipment safety and reliability to studying means of strengthening equipment for harsh environments. The examples on display here are just a few of the hundreds of pieces of equipment NEDU has evaluated and certified for Navy use.


Surface-Supplied Diving Systems

A surface-supplied diving system provides breathing air to a diver through an umbilical (hose) connected to control equipment on the surface. Most Navy diving operations use surface-supplied systems. In the early 1970s, NEDU initiated a project to modernize the Mark V system, in service since 1916. NEDU developed, tested, and certified a new system, the Mark 12 Surface-Supplied Diving System (SSDS), which was adopted in 1985. NEDU also led the subsequent modernization effort, evaluating and adapting a commercial helmet for use as the Mark 21 helmet.

Mark 12 Surface-Supported Diving System

NEDU designed the Mark 12 after identifying a need for an updated diving system. Following a four-year development period, NEDU conducted three years of testing before the system was approved for use. The Mark 12 provided divers with greater mobility, flexibility, and comfort compared to the retired Mark V.

Mark 21 Helmet

The Mark 21 helmet replaced the Mark 12 system in 1993. Instead of designing a system, the Navy chose to adapt a commercial helmet, the Superlite 17B, to Navy specifications. NEDU conducted the analysis of the helmet, identified changes needed to meet Navy standards, and certified the modified design. Unlike its predecessors, the Mark 21 system does not include a standard diving suit. It can be used with different types of suits and gear.

Mark 14 Closed-Circuit Saturation Diving System

This saturation diving system operates from an underwater saturation diving bell, rather than from the surface. NEDU developed it in the 1970s to support saturation diving projects. The system improved the Navy’s saturation diving capabilities by streamlining the equipment needed, reducing costs, and increasing work time underwater.


Diving Masks

The Navy uses specialized diving masks for some types of surface-supplied diving. In warm, uncontaminated environments, diving masks offer a lightweight alternative to the typical surface-supplied system. NEDU tests commercial masks for Navy use, recommending safety modifications and certifying Navy mil-spec versions.

Jack Browne Mask

The Navy used the Jack Browne lightweight mask for shallow water diving from World War II through the late 1970s. NEDU evaluated the mask on multiple occasions. While assessments in the 1950s affirmed its usefulness, by 1978 NEDU determined the aging mask could no longer support intensive diving operations. The Navy limited its use to 40 feet before retiring it in the 1980s.

Mark 1 Mask

The Mark 1 lightweight mask supported both air and mixed gas operations, could be used to 300 feet, and provided voice communications. It is a modified version of the commercial Kirby Morgan band mask, which NEDU tested in the early 1970s. The Navy adopted it after modifications recommended by NEDU’s assessment were implemented.



NEDU began experimenting with rebreathers as early as World War II. A rebreather is a type of underwater breathing apparatus (UBA) that recycles a diver’s breathing mixture by scrubbing it of carbon dioxide. Navy EOD and Special Forces use rebreathers for covert operations. Compared to surface-supplied diving equipment, rebreathers are more compact, offer greater mobility, and do not produce visible gas bubbles. NEDU’s work with rebreathers has significantly expanded the capabilities of EOD and Special Forces divers.

Mark 11 Rebreather

NEDU evaluated the Mark 11 rebreather in the 1970s, testing everything from overall unit safety to the life expectancy of the carbon dioxide scrubbing canisters.

Mark 15 Rebreather

After the Navy purchased the Mark 15 rebreather commercially in 1980, NEDU tested and certified it for use by Navy Special Forces. NEDU also developed new decompression tables to use with the Mark 15, as the device’s ability to keep the oxygen level in the breathing mixture constant meant standard tables could not be used.

Mark 16 Rebreather

Following successful testing by NEDU in the early 1980s, the Navy adopted the Mark 16 rebreather (an improved version of the Mark 15) to support Explosive Ordnance Disposal (EOD) operations. The Mark 16 rebreather’s low magnetic output keeps EOD divers safe when performing work that might involve magnetically-detonated mines.


Diving Suits

Divers frequently work in cold, contaminated, or physically hazardous environments. Since introducing the first neoprene suits in the early 1950s, NEDU has developed and tested improved diving suits to provide physical and thermal protection to divers.

Wet Suits

Post World War II, NEDU faced the challenge of keeping divers warm. Staff repurposed an aviator’s neoprene exposure suit, which had its tester perspiring 1.5 hours into a cold dive. Through additional experimentation, NEDU tested neoprene thickness and overall design to maximize thermal and physical protection.

Hot Water Suits

Thermal protection is especially critical at greater depths (water becomes cold with depth), when breathing helium (which draws warmth from the diver’s body), or during saturation dives (which last several hours at deep depths). Hot water suits continually bathe divers in hot water from the surface, keeping them toasty warm.