The Evolution of US Combat Diving. Part 2, the Re-breather.

Exploring the evolution of combat diving.

This is Part 2 of 7 taken from my Submarine Medical Officer Thesis “A Review of the Combat Diving Evolution of US Naval Special Warfare with a Focus on the Necessity of Disruptive Technologies and Innovative Scientists.” Stay tuned for part 3 next week.

The Closed-Circuit Underwater Breathing Apparatus

Closed-circuit UBAs, now used widely in special operations, have unique attributes that make them especially suited for the work of the combat diver. Unfortunately, the UDTs did not employ diving technology until almost a decade into the worldwide combat diver evolution, but it was only after adopting and implementing this capability that the UDT/SEALs truly became fully functional maritime warriors.

Combat Swimming. During WWII, the UDTs typically performed beach reconnaissance and obstacle demolition with nothing more than a pair of their iconic UDT shorts, a knife, a mask, a satchel of explosives, and later a pair of fins (Fig. 2); thus earning the moniker “The Naked Warriors.” They spent very little time underwater in WWII other than during a “breath hold”, essentially serving as purely “combat swimmers.”1,2 The classic UDT mission began with a high speed boat traveling parallel to the beach about a thousand meters from the shore. The UDTs rolled into the ocean, formed a swimmer line, recorded depths to the high water line, returned to sea, and extracted by the same high speed boat using a unique “snaring” maneuver. Similar insertions and extractions were used for a variety of missions, including those involving demolitions. This method would never be utilized in modern non-permissive environments, but it worked in WWII due to the nature of warfare during that period.3,4

Figure 2: WWII UDT placing demolitions on an underwater obstacle.5

In WWII “island-hopping,” an entire fleet staged offshore an enemy occupied coast while the UDTs performed preliminary reconnaissance ahead of the amphibious assault force. Since performing the reconnaissance clandestinely was impossible at that point, the classic overt method sufficed. But the UDTs were essentially unprotected and highly vulnerable to attack. Some precautions reduced risks including the use of deception, security provided by the insertion boats, and missions conducted at night, but when the UDTs were discovered, they frequently came under fire from the enemy shore. Luckily, the UDTs found a haven resting low in the water and did not suffer high casualty rates, but discovery by the enemy was highly problematic.6,7,8 Eventually, converting combat swimmers to combat divers would solve this problem.

Combat Diving. Until WWII, combat diving was impossible due to limited technological capabilities. At that time, fleet Navy divers practiced surface-supplied diving – also called “hard hat” diving – from ships with large banks of compressed air and teams of divers for support. Navy divers wore the iconic Mark V helmets, heavy dive suits, and weighted boots to lend stability on the ocean floor. Although these attributes were ideal for Navy divers, they made Navy diver equipment incompatible with combat diving. Eventually, the invention of the “self-contained” UBA (SCUBA) gave combat divers the independence and mobility necessary for their work.9

Various other nations and organizations utilized UBAs in WWII combat operations including the Italians, the British, the Japanese, and the Office of Strategic Services. The Italian “Gamma Men” – the renaissance men of combat diving – wore closed-circuit UBAs while riding and guiding torpedoes10 underwater, undetected to destroy multiple British ships in the Mediterranean as early as 1941. The British used similar tactics to successfully attack the Tirpitz – a German Battleship – and many other ships throughout WWII. Later in the war, with only marginal success, the Japanese executed “Kamikaze” attacks with their own “Kaiten diver-guided torpedoes.”11,12

The LARU and the AquaLung. The US Office of Strategic Services (OSS) – predecessors to the CIA – was created in 1942 and promptly established its own Maritime Unit (MU) to conduct secretive waterborne missions during WWII. The MU men are considered “the first underwater combat and sabotage forces in the US Military.”13 In 1939, Christian Lambertsen, who later became a MU member, developed the Lambertsen Amphibious Respiratory Unit (LARU, see Fig. 3) – the first US closed-circuit 100% oxygen SCUBA14 – while studying as a medical student at the University of Pennsylvania. After designing and building the first LARU, he personally dove the experimental rig multiple times in Lake Nokomis in Minnesota before he was confident in its functionality and reliability.15,16,17,18

Lambertsen initially introduced the LARU to the Navy Experimental Diving Unit (NEDU) in 1940, but Navy Divers have much different dive profiles and requirements than “combat divers,” so they showed no interest.19,20,21 However, the OSS MU divers quickly adopted the LARU after observing that its design was far superior to other contemporary closed-circuit rebreathers.22 In 1943, some of the OSS MU men integrated with the UDTs and demonstrated their new LARUs eliciting initial excitement. However, the LARU was eventually rejected by UDT leadership for nebulous reasons, dooming the UDTs to inferior technology and tactics for years. The LARU was utilized throughout WWII by the OSS MU, but did not see combat despite numerous successful training operations.23,24 Interestingly, although the UDTs are considered the predecessors to the SEALs, the OSS MU was much more similar to the modern SEAL Teams with respect to their equipment, skills, and operational capabilities.25 Unfortunately, the OSS was decommissioned after WWII and all of its developments and technologies were archived and retired – a terrible waste of resources and years of knowledge and experience.26

Figure 3: Dr. Christian Lambertsen’s LARU.27

In 1947, the evolution of radar technology forced the UDTs to reconsider diving and the utilization of the LARU. LCDR Douglas Fane – a UDT commander known as “Red” – sought out Dr. Lambertsen after one of his superiors recommended, “For the safety of your men, if for no other reason, you will have to go underwater. Operations from submarines are the only possible way of conducting secret UDT missions.”28 Lambertsen facilitated Fane’s request for help, and soon recalled the LARU from retirement. In 1948, because Dr. Lambertsen was the LARU subject matter expert, he accompanied LCDR Fane in locking out of a submarine – the USS Grouper – for a thirty minute LARU dive at periscope depth before locking back into the sub.29 Lambertsen later recalled, “There was a sensation of tremendous speed…I gingerly experimented with turning my head a little to one side. The downstream edge of the mask began to vibrate, but no water leaked in, so I was able to see Fane. His red hair was streamed back, his ears were flapping in the submarine breeze, his trunks and lung were bellied out behind and whipping about. I could think of no better simile than Ben Hur driving his chariot in his race around the Roman Coliseum.”30 After initial testing and evaluation, the UDTs utilized the LARUs on a limited basis, and created a “submersible operations” platoon to further develop the capabilities of this novel combat diving concept.31,32

During this time, the UDTs also discovered the AquaLung, a new French invention similar to modern recreational SCUBA rigs. The AquaLung was easy to use and maintain which lead to its adoption and the replacement of the LARUs. Sadly, the LARU was discarded due to its aging and irreplaceable parts and the still mysterious associated negative effects – oxygen toxicity and the “caustic cocktail.”33 However, the AquaLung was an open-circuit SCUBA that could never be effectively used operationally due to its heaviness, bulkiness, and “telltale” bubbles. The adoption of the AquaLung essentially halted the technological progression of the “re-breather,” and stalled the evolution of US Navy combat diving for another decade.34

The MK 25 UBA. In 1962, after the creation of the SEALs, there was a revived interest in “bubble-less” diving systems.35 Over the next twenty years, various diving rigs were tested, revisions were made, faulty designs were discarded, and eventually, in 1982, the Navy adopted the Draeger Lung Automatic Regenerator (LAR V, or MK 25, see Fig. 4&5) which is still used extensively by US combat divers.36 The MK 25 MOD 2 – a newer, upgraded version – is a light-weight, chest mounted, closed-circuit, 100% oxygen “re-breather” similar to Dr. Lambertsen’s LARU from the early 1940s. Its mechanically simple operation is composed of a completely enclosed, circulating system that provides oxygen (O2) via a pressurized O2 bottle and eliminates carbon dioxide (CO2) via a CO2 absorbent canister. Since its adoption, the MK 25 has successfully proven itself due to its simplicity and reliability, but the combat diver must operate carefully within specified parameters to avoid dangerous potential physiologic catastrophes.37,38,39

Figures 4 & 5: The Draeger LAR V UBA, or MK 25.41

The Re-Breather Benefits. There are three main reasons for the re-breather’s eventual, necessary adoption and widespread use – its bubble free operation, its extended dive time, and its relatively minimal weight.42

First, the greatest benefit of a closed-circuit UBA is the absence of emitted bubbles during its use. When using “open-circuit” diving systems, every dive supervisor watches for a trail of bubbles that tracks a diver’s underwater progress. A trained eye can spot these bubble trails in rough seas from a moderate distance. But “telltale” bubbles can be catastrophic for a combat diver, and bubbles are completely incompatible with their secretive missions. Since a closed-circuit UBA does not generate bubbles, a combat diver can deploy from a submerged submarine, swim underwater to an enemy beach, conduct necessary reconnaissance, and return to the offshore submarine without ever surfacing – effectively accomplishing the same WWII missions but without being detected.43,44

A re-breather also provides additional dive time and nautical range due to the system’s recycling mechanism. In an open-circuit diving system, every exhale is exhausted into the ambient underwater environment producing a wasted cloud of bubbles. Earth’s environment is composed of 21% oxygen, and we exhale 16% oxygen. Therefore, we only effectively use 5% of the surrounding air to feed our metabolism, or only 24% of the available oxygen – an inefficient system. With a closed-circuit re-breather, every exhale is recycled. The carbon dioxide produced by the body is scrubbed, additional oxygen is injected, and the diver breathes 100% oxygen again45 – making this a much more efficient system than open-circuit SCUBA. This increased efficiency allows for a lengthier dive time, and thus more distance can be covered. A common open-circuit SCUBA tank will give a typical diver twenty to forty minutes of dive time in average diving conditions; but a small, chest strapped, MK 25 closed-circuit “re-breather” affords the combat diver many hours of dive time under the same conditions.46,47

Third, the MK 25 re-breather is comparatively lighter than open-circuit dive systems. The weight difference is not necessarily an advantage when submerged, but on land, lighter diving rigs offer a significant benefit for maneuvering and carrying a combat loadout over the beach.48 This can be a remarkable advantage for the combat diver whose first steps on land may be after a lengthy, exhausting dive.

The Re-Breather Problems. Despite these advantages, there are also problems with 100% oxygen re-breathers that can potentially cause death. When breathing 100% oxygen at depth, under certain conditions the diver can experience a physiologic phenomenon known as oxygen toxicity. And there are two different kinds of oxygen toxicity – central nervous system (CNS) oxygen toxicity and pulmonary oxygen toxicity. Both toxicities occur when breathing supra-physiologic partial pressures of oxygen at either deeper depths or for longer periods than recommended.

CNS O2 toxicity has many presentations – visual disturbances, tinnitus, nausea, muscle twitching, irritability, dizziness – but the most dangerous sequelae is a seizure. Seizures underwater can understandably result in the death, so CNS O2 toxicity must be avoided at all costs. Unfortunately, the manifestation of CNS O2 toxicity varies individually and episodically. There are no predictable auras or symptom complexes that invariably progress to convulsions, so there are no reliable methods to predict who will experience a seizure or when. So, seizures must be avoided at all costs. And this is accomplished by restricting dive depths and dive times utilizing safety limits (Table 1) published by the Navy Experimental Dive Unit (NEDU). These measures largely reduce the risk of a diver developing O2 toxicity, but these restrictions also limit the MK 25 to shallow operating depths with a few exceptions – for brief excursions below the keel of a ship or under obstacles (Table 2). But since SEALs mainly use this rig for transiting from one area to another at shallow depths, these restrictions are not a significant problem.49,50,51,52

Table 1: Current depth and time limits on 100% O2 Re-breathers.53
Table 2: Excursion limits for 100% O2 re-breathers.54

The MK 16 UBA. The MK 16 UBA (Fig. 6&7) is another closed-circuit re-breather utilized by NSW. The MK 16 differs from the MK 25 primarily in that it is a mixed-gas UBA rather than a 100% oxygen rig. The MK 16’s gas mixtures – air (N2O2) and heliox (HeO2) – are controlled automatically by three redundant sensors to maintain a constant partial pressure of oxygen regardless of depth. The mixed-gas operation allows SEALs and Explosive Ordnance Disposal (EOD) divers to dive much deeper55 than with the MK 25 by avoiding the high partial pressures of oxygen and the resulting oxygen toxicity.  The MK 16 is a “backpack-like” re-breather, and has similar benefits to the MK 25 re-breather including bubble-free operation and increased dive time.56

The Impact of the Re-breather. With the MK 25, MK 16, and open-circuit SCUBA, the SEALs currently employ a versatile diving arsenal that enables them to operate undersea stealthily, over long distances, and at a variety of depths. Although initially hesitant to join the combat diver evolution, and despite multiple diversions along the way, the UDT/SEALs eventually realized the importance of diving and invested immense time and effort into developing this capability. Interestingly, evolving technologies forced the UDTs to operate underwater, but evolving technologies also allowed them to dominate that realm. Now with the modernization and improvement of “re-breathers,” SEALs have nearly realized the full potential of combat diving. By adapting to modern technologies and tactics and then exploiting the advantages of closed-circuit diving rigs, the SEALs greatly enhanced their survivability and mission capabilities. But the US combat diver capability remained incomplete until the development of another ingenious technology – the swimmer delivery vehicle (SDV).

Continued in Part 3…


  1. The terms “combat swimmer” and “combat diver” are used interchangeably, but in this study “combat swimmers” will refer to those who work primarily on the surface (early UDTs) and “combat divers” will refer to those who work primarily underwater (later UDTs and modern SEALs).
  2. Orr Kelly, Brave Men Dark Waters: The Untold Story of the Navy SEALs (Novato, CA: Presidio, 1992), 18, 41
  3. Tim Bosiljevac, SEALS: UDT/SEAL Operations in Vietnam. (Boulder, CO: Paladin Press, 1990), 2-5.
  4. Kevin Dockery, Navy SEALs: A Complete History from World War II to the Present (New York: Penguin, 2004), 83-86, 94,102-104.
  5. WWII UDT Placing Demolitions on Underwater Obstacle, picture, accessed December 19, 2015,
  6. Bosiljevac, SEALS, 2-5
  7. Jim Joiner, editor, Naval Forces Under the Sea: A Look Back, A Look Ahead (Flagstaff, AZ: Best Pub. Co., 2002), 2-49
  8. Dockery, Complete History, 83-86, 94,102-104.
  9. Mark V. Lonsdale, United States Navy Diver: Performance Under Pressure (Flagstaff, AZ: Best Pub. Co., 2005), 41-42, 49-51.
  10. Italian “torpedoes” were motorized mines shaped similar to modern torpedoes but designed to be ridden and guided by combat divers. The divers placed the torpedo under the ship where it would be detonated by a timer after the divers were at a safe distance. Japanese diver-guided torpedoes were actually “kamikaze” suicide devices.
  11. Patrick K. O’Donnell, First SEALs: The Untold Story of the Forging of America’s Most Elite Unit (Boston: Da Capo, 2014), 5-8
  12. United States Navy Diving Manual, Revision 6 (Washington D.C.: US Government Press, 2008), 1-13-14.
  13. Dockery, Complete History, 92.
  14. Dr. Christian Lambertsen coined the term “Self-Contained Underwater Breathing Apparatus” (SCUBA).
  15. O’Donnell, First SEALs, 1-3
  16. Navy Diving Manual, 1-10-14
  17. Dockery, Complete History, 92-93
  18. Butler FK. “Closed-circuit oxygen diving in the U.S. Navy,” Undersea Hyperbaric Medicine 2004; 31: 3-20.
  19. Joiner, Naval Forces: A Look Back, 2-8.
  20. Kelly, Brave Men, 46
  21. Butler, “Closed-circuit Diving,” 3-20.
  22. Butler, “Closed-circuit Diving,” 3-20.
  23. Dockery, Complete History, 93
  24. O’Donnell, First SEALs, 41-44, 77-78.
  25. O’Donnell, First SEALs, 136.
  26. Tucker and Lamb, Special Operations Forces, 79-80.
  27. Navy Diving Manual, Figure 1-11.
  28. O’Donnell, First SEALs, 236.
  29. Butler, “Closed-circuit Diving,” 3-20.
  30. O’Donnell, First SEALs, 236-237.
  31. Ibid.
  32. Navy Diving Manual, 1-14-15.
  33. Water infiltrates the CO2 absorbent canister and causes the release of caustic gases that cause chemical burns.
  34. Kelly, Brave Men, 55-58.
  35. Ibid., 59-61.
  36. Navy Diving Manual, 1-14.
  37. Jim Joiner, editor, Naval Forces Under the Sea: The Rest of the Story (Flagstaff, AZ: Best Pub. Co., 2007), 52
  38. Navy Diving Manual, 1-14
  39. Alfred A. Bove, Diving Medicine, 4th edition (Philadelphia: Saunders, 2004), 551-557.
  40. Navy Diving Manual, Figure 19-1, 19-3.
  41. Navy Diving Manual, Figure 19-1, 19-3.
  42. Navy Diving Manual, 19-1-25.
  43. Robert A. Gormly, Combat Swimmer: Memoirs of a Navy SEAL (New York: Penguin, 1999), 5-21
  44. Navy Diving Manual, 19-1-25.
  45. Although referred to as a 100% O2 rig, the actual oxygen fraction in the MK 25 system ranges from 71-82% at the range of operating depths. To avoid digressing into minutiae, this study will continue to refer to the MK 25 as a 100% oxygen rig due to oxygen being the only supplied gas.
  46. Navy Diving Manual, 19-1-25
  47. Bove, Diving Medicine, 549-552
  48. Navy Diving Manual, 19-1-25.
  49. Joiner, Naval Forces: The Rest, 77
  50. Navy Diving Manual, 3-41-45, 19-1-25
  51. Bove, Diving Medicine, 548-562
  52. Butler, “Closed-circuit Diving,” 3-20.
  53. Navy Diving Manual, Table 19-4.
  54. Ibid., Table 19-3, Figure 19-4.
  55. Down to 300 feet of sea water on heliox.
  56. Ibid., 17-1-11, 18-1.
  57. EOD Diver Working on an Underwater Mine, picture, accessed December 19, 2015, wikipedia/commons/9/9e/ Aa_US_Navy_explosive_ordnance_disposal_(EOD)_divers.jpg
  58. Navy Diving Manual, Figure 17-4.

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