The historical record is awash with examples of early peoples seeking to explore the underwater world. In a section of the ancient Greek text Problemata, which may or may not have been written by Aristotle around 360 B.C., the author hypothesizes the use of a kind of diving bell, an inverted “kettle” filled with air to give sponge divers an underwater base of operations for extended dives.

Leonardo da Vinci, for one, claimed to have figured out how a person could remain submerged for an extended period of time – but also claimed he would never publish the details of this information, “because of the evil nature of men who practice assassination at the bottom of the sea.”

During Alexander the Great’s siege of Tyre in 332 B.C., enemy divers continually severed the mooring ropes of Alexander’s ships and set them adrift to crash into each other. Though no record of the siege mentions the use of a diving bell, a legend emerged of Alexander being lowered into the harbor in a glass barrel or jar for several minutes to observe the goings-on. For centuries thereafter, versions of this tale were celebrated in texts and paintings from Britain to India.

One of the first actual uses of the diving bell was recorded by Francesco de Marchi of Bologna, who, in 1535, used a primitive one-person diving bell to explore the sunken wrecks of the Emperor Caligula’s fabled Lake Nemi ships. By now, the Western world’s leading thinkers had begun to envision a kind of underwater boat that could move under propulsion. Leonardo da Vinci, for one, claimed to have figured out how a person could remain submerged for an extended period of time – but also claimed he would never publish the details of this information, “because of the evil nature of men who practice assassination at the bottom of the sea.”

In 1775, the young American David Bushnell, with encouragement from both Benjamin Franklin and George Washington, devised the Turtle to attack the British warships blockading colonial ports.

In 1578, seven decades after da Vinci’s death, the English mathematician William Bourne published his own idea for a submersible in the book Inventions or Devices, which included a description of “a shippe or boat that may goe under the water unto the bottome, and so come again at your pleasure.” Though he included no drawings or models, Bourne described how the craft – essentially a wooden boat covered in oiled leather – could be raised or lowered in by filling and emptying ballast tanks, and how its occupants could breathe by means of a hollow mast protruding upward.

The first submersible boats to be made to Bourne’s description were conceived by Dutch physician Cornelius Drebbel, who tutored the children of King James I and served as “court inventor.” While Bourne had hypothetically solved the problems of buoyancy and air supply, Drebbel added a solution to how the boat could be propelled: A crew of oarsmen, if the boat were properly sealed and ballasted, could drive it. Few records of Drebbel’s design remain, but he built and successfully tested at least three of these submersibles – the largest of which carried 16 passengers and was demonstrated in front of King James and several thousand spectators. The boat stayed submerged for three hours, cruising at a maximum depth of about 15 feet.

Drebbel’s invention impressed King James – who rode along for a test dive beneath the Thames – but England’s Royal Navy reacted to these demonstrations with indifference, establishing an unfortunate precedent. For the next three centuries, while the English continued to dismiss the submarine’s potential, their enemies developed and refined the submarine as a means of attacking the world’s most powerful navy. In 1775, the young American David Bushnell, with encouragement from both Benjamin Franklin and George Washington, devised the Turtle to attack the British warships blockading colonial ports. Small and cumbersome, propelled by two screw propellers, the Turtle proved too difficult to operate; it failed in its mission to blow up the HMS Eagle in New York Harbor and was later sunk. It was, however, the first documented use of a submarine in combat.

The next great innovator in submarine development was the Irish-American artist and engineer Robert Fulton, who spent many years in Europe and grew to loathe the Royal Navy – which he, an Irish nationalist, believed was choking off freedom and commerce around the world. By the late 1790s, Fulton had developed plans for an undersea boat he called the Nautilus. Sheathed in copper over iron ribs, the Nautilus was a cigar-shaped craft, 21 feet long and more than 6 feet at the widest, powered by a hand-cranked propeller. Horizontal fins controlled the angle of dive, and a hollow iron keel served as its ballast tank. Above deck, the Nautilus had several new features – a fan-shaped sail that could be deployed to help propel the boat when surfaced; a periscope that would allow an underwater observer to see above the horizon; and a small observation dome that presaged the modern conning tower. Bottled, compressed air allowed the manned craft to remain submerged for up to five hours, and a snorkel could be extended to supplement this supply.

The Nautilus was designed to carry an explosive charge Fulton called a “carcass,” also commonly known as a “torpedo,” that could be attached to the hull of a ship and detonated from a distance, making it an ideal weapon to break the Royal Navy’s blockade of France. After successful demonstrations of the Nautilus in the Seine and the English Channel, Fulton offered to make submarines for the French – who declined, for both practical and moral reasons. A human-powered submarine was simply too slow, and its range too limited, to be useful in naval combat – and the French Ministry of the Marine considered the submarine an underhanded tactical weapon, fit for pirates. Fulton lent credence to this idea when he asked for himself and his men to be commissioned as officers in the French navy, officially recognized as belligerents, to avoid being executed if they were captured.

Rebuffed by the French, Fulton apparently shrugged off his hatred of the British and offered to sell his plans to Prime Minister William Pitt, who encouraged a public demonstration of the stealth attack. This kind of warfare was promptly denounced by other Britons as cowardly, an attitude later summed up by John Jervis, 1st Earl of St. Vincent and admiral of the fleet: “[Pitt] is the greatest fool that ever existed to encourage a mode of war which they who commanded the seas did not want and which, if successful, would deprive them of it.”

With no prospects in Europe, Fulton returned to the United States and, in 1810, persuaded Congress to put up $5,000 for a steam-powered submarine that, if he’d lived to complete it, would have represented a revolution in submarine technology. As it was, many problems remained to be solved before the submarine could serve reliably as a naval warship.

Propulsion and Weaponry: Solving the Submarine

Circumstances had led the submarine’s inventors to envision it as a vehicle to be used primarily as a tool of war. By the end of the 19th century, it could only be imagined as a defensive tactical weapon, to surprise and check a more powerful enemy in coastal and harbor defense. Its stealth was still widely disdained. In 1901, British Adm. Sir Arthur Wilson declared the submarine “underhand, unfair and damned un-English.”

The two primary weaknesses of early submarines were their unreliable – and dangerous – weaponry and their sluggish means of propulsion. By 1870, the crude “torpedo” had been refined by the Englishman Robert Whitehead, who had developed an unguided, self-propelled torpedo that could be fired from a launching tube. In 1885, the Swedish gun maker Thorsten Nordenfelt introduced a submarine, the Nordenfelt I, fitted with a deck-mounted torpedo tube. Its steam-powered engines made the Nordenfelt more of a semi-submersible than a submarine; the heat, smoke, and exhaust from combustion rapidly accumulated inside the hull, prompting frequent surfacing.

Holland had studied the Civil War exploits of the ironclads Monitor and Merrimack and the Confederate submarine H.L. Hunley – the first combat submarine to sink a warship – and had come to believe, like Fulton, that the submarine would be key to breaking the back of the hated Royal Navy.

The French submarine Narval, launched in 1899, was the first to use two different propulsion systems: an oil-fired steam engine on the surface, and an electric motor when submerged. The steam engine served as a dynamo, recharging the electric motor’s batteries – a refinement that would be imitated for decades to come. Steam engines, however, were notoriously unreliable; their bulky boilers were prone to explosion, and had to be shut down and sealed off from the outside before the submarine could submerge.

The next great submarine innovators set their sights on a more stable and reliable means of propulsion, and the one who stood out from the crowd – who created what military historian Thomas Parrish, in his book The Submarine: A History described as “the world’s first functional, operational, nonexperimental submarine” – was John P. Holland, an Irish schoolmaster who emigrated to the United States in 1873. Holland had studied the Civil War exploits of the ironclads Monitor and Merrimack and the Confederate submarine H.L. Hunley – the first combat submarine to sink a warship – and had come to believe, like Fulton, that the submarine would be key to breaking the back of the hated Royal Navy.

The crowning achievement of Holland’s work, the Holland IV, was launched in 1897. More than 53 feet long and 10 feet at the widest, it was propelled on the surface by a recent innovation – gasoline-powered internal combustion engines – that gave it a surface range of 1,000 miles at a top speed of 8 knots, and could be used to charge an electric motor that could send the vessel 30 miles submerged on a single charge. The submarine was remarkably maneuverable: It dove, for example, under its own power, rather than waiting for ballast tanks to fill and empty. An improved version of the Holland boat became SS-I, the U.S. Navy’s first commissioned submarine, in 1900.

One subsequent improvement substituted cleaner-burning, less-volatile diesel engines as power sources, making diesel-electric submarines the standard for decades to come.

The submarines and unterseeboote (U-boats) of the world wars were, for the most part, variations on the Holland design. One subsequent improvement substituted cleaner-burning, less-volatile diesel engines as power sources, making diesel-electric submarines the standard for decades to come. All relied – even with the invention of the snorkel, which drew air into the diesel engines and charged the boat’s battery pack by means of a deployable snorkel mast – on air supply for combustion, and operated primarily as surface ships that could submerge for a time when escaping or attacking. This remained the case until the latter years of World War II, when the German Kriegsmarine began producing the Type XXI U-boats or elektroboote. The Type XXI U-boat was designed to spend its entire patrol – more than 17,000 miles – submerged, employing a snorkel to run its diesels or running off its huge battery array, and this operational refinement led to a more streamlined hull configuration and the removal of deck hardware to optimize underwater speeds.

At the turn of the 20th century, it was obvious that the world’s navies had been presented with a formidable weapon. The question was: What to do with it? For the next half-century, to some extent, world history was written by the varied answers to this question – and those answers revealed a naval culture and strategic thinking that had not yet caught up to the submarine’s disruptive influence. The British Royal Navy, it turned out, had no idea what a submarine was for; to the Admiralty, it remained an ungentlemanly nuisance, if not a piratical outrage. All World War I combatants, at the outset, envisioned the role of a navy as a large flotilla that would meet its adversary on the open sea, as at Trafalgar, and set guns blazing until a victor emerged.

The smaller German navy, however, after learning it couldn’t break the Royal Navy’s blockade, promptly switched tactics, unleashing its U-boats to torpedo and sink merchant ships that kept the island nation supplied. This approach was roundly condemned, and prompted debate over whether submariners at war should observe the previous century’s “prize” or “cruiser” rules for wartime ship captures, which required the safe evacuation of crew and passengers before a ship was captured or sunk.

The Type XXI U-boat was designed to spend its entire patrol – more than 17,000 miles – submerged, employing a snorkel to run its diesels or running off its huge battery array, and this operational refinement led to a more streamlined hull configuration and the removal of deck hardware to optimize underwater speeds.

Prize rules were impracticable for submarines. When the German U-20 carried out the most notorious naval attack of the war, sinking the ocean liner RMS Lusitania with a torpedo off the coast of Ireland on May 7, 1915, it was denounced as a war crime on both sides of the Atlantic: The Lusitania, briefly the largest passenger liner in the world, had been carrying 1,959 passengers and crew, and 1,198 of them – noncombatants all – lost their lives.

The German perspective differed: The ship had also departed New York with more than 173 tons of munitions for the British, making it a necessary target. The Germans’ continued practice of unrestricted submarine warfare, however, would be an important factor in the U.S. decision to enter the war on the Allied side.

Powered by an inexhaustible supply of steam heated by a nuclear reactor, Nautilus was the first submarine to use a safe and reliable means of air-independent propulsion (AIP) – a technology that had consisted, to date, of experimental closed-cycle combustion engines fed by bottled or chemical sources of oxygen that were inherently unsafe.

Having begun each world war on the defense against superior German submarines, the United Kingdom and its allies ramped up efforts to counter the threat, developing technologies and methods that would be known collectively as anti-submarine warfare, (ASW): Underwater microphones, or hydrophones, along with the active sound detection known as ASDIC, later refined by American researchers into sonar, provided an early but crude means of detection. The depth charge – an explosive with a hydrostatic switch that would detonate it at a specified depth – was first used successfully on March 22, 1916, when HMS Farnborough – a “Q-ship,” or armed merchant vessel designed to bait a U-boat into attacking – sank the German U-68 off the coast of Ireland.

The submarine continued to be a tool whose uses were interpreted differently by World War II combatants. The Imperial Japanese Navy’s attack on Pearl Harbor included a force of five midget submarines, transported on the decks of larger subs. The effectiveness of these submarines during the attack is still debated today, but their use illustrates the rapidly evolving state of submarine warfare at the mid-20th century.

The Nuclear Age

Questions concerning the ethics of submarine warfare were rendered quaint-sounding by the United States’ use of atomic bombs on Hiroshima and Nagasaki in August 1945. Years later, the man who would become the architect of the U.S. Navy’s nuclear fleet, Hyman G. Rickover, expressed to Congress the dangers of planning for wars fought in the old way: “The lesson of history,” he said, “is that when a war starts every nation will ultimately use whatever weapon it has available.”

In 1955, Rickover’s advocacy helped yield the world’s first nuclear-powered submarine, the USS Nautilus (SSN 571), signaling not only a new era in submarine technology and defense doctrine, but also a dramatic shift in Cold War geopolitics. Powered by an inexhaustible supply of steam heated by a nuclear reactor, Nautilus was the first submarine to use a safe and reliable means of air-independent propulsion (AIP) – a technology that had consisted, to date, of experimental closed-cycle combustion engines fed by bottled or chemical sources of oxygen that were inherently unsafe.

In theory, the Nautilus could stay submerged indefinitely, but the 1,800 miles it traveled under the Arctic ice in the summer of 1958, from the Bering Strait to the eastern coast of Greenland, was enough to tip the balance of the Cold War. To the Soviets, who, at the time, enjoyed a 450-to-110 advantage over the United States in military submarines – and who had just a year earlier shocked the world with the launch of the Sputnik I satellite – the unspoken message of the Nautilus voyage was clear: The United States had a vessel that could travel unchecked to the 3,000-mile-long Murmansk-to-Vladivostok coastline of the Soviet Union – a prospect made terrifying by the U.S. Navy’s successful submarine launches of Regulus nuclear-armed guided cruise missiles and, in 1960, of the Polaris submarine-launched ballistic missile. What the U.S. fleet lacked in numbers, it made up in technological superiority; despite Soviet advances, the U.S. Navy enjoyed this advantage through the end of the Cold War – and to the present day.

The ensuing half-century was a race for technological improvements in submarine and ASW design, and yielded innovations such as acoustic dampening techniques; sophisticated sensors, fire-control systems, and electronic support arrays; and unmanned underwater vehicles, or UUVs, which proved capable of reaching the ocean’s greatest depths. The question that had never really been resolved during World Wars I and II – What’s a submarine for? – was answered by the Cold War: A nuclear submarine was a powerful deterrent, an indispensable component in the doctrine of mutually assured destruction. Its stealth, in addition, provided a platform for SIGINT, or the gathering of intelligence through interception of analog or electronic communication signals. These roles – projecting power and gathering intelligence – have remained at the core of U.S. naval doctrine since the launch of the Nautilus.

The 21st Century: The Emerging
“Cyber Sub” Era

The post-Cold War era presents a much more complex strategic arena. China’s demonstrated ambition to build a blue-water navy, the re-emergence of Russian military might, and a burst of innovation in information technology are among the factors that have made the future of undersea warfare increasingly uncertain.

While the United States’ Virginia-class submarines are the most advanced undersea warships ever built, the gap is arguably closing; as other nations use powerful processing technologies and detection capabilities to extend the range and effectiveness of anti-access/area denial (A2/AD) capabilities, some are also developing super stealthy non-nuclear submarines: Sweden’s Gotland-class submarines, for example, are the first in the world to feature a Stirling engine AIP system, allowing them to remain underwater for weeks. The German Type 212 submarine is a diesel/fuel-cell hybrid that can stay submerged for up to three weeks. Both submarines can operate more quietly than nuclear submarines.

At the same time, quantum leaps in sensing and computing power have moved ASW beyond the capabilities of active and passive sonar; it’s widely expected that optical sensors, composed of LED-transmitted beams or lasers, will soon operate at greater range and sensitivity than low-frequency sonars.

At the same time, quantum leaps in sensing and computing power have moved ASW beyond the capabilities of active and passive sonar; it’s widely expected that optical sensors, composed of LED-transmitted beams or lasers, will soon operate at greater range and sensitivity than low-frequency sonars. Passive sensors are capable of monitoring changes in the ocean environment, such as changes in current, radiation, ambient noise, or surface disruptions, that signal the presence of an underwater craft. Getting close to a country’s shoreline or its naval assets, undetected, is likely to become more difficult, posing greater risks for traditional manned submarine operations.

All of which has led some observers, such as Bryan Clark of the Center for Strategic and Budgetary Assessments, to see submarine warfare on the brink of a 21st century transformation. In “The Emerging Era in Undersea Warfare,” published in January 2015, Clark, a former submariner and special assistant to the Chief of Naval Operations, foresees a future in which increasingly expensive and vulnerable manned submarines remain at a distance, dispatching both UUVs and unmanned aerial vehicles (UAVs) to do increasingly nonlethal work. In the Information Age, the ability to destroy or degrade an adversary’s networking or signaling capabilities may be more important than firing missiles. The earliest experimental aerial and undersea drones have been fired from modified missile or torpedo tubes; in a 2013 evaluation, the U.S. Naval Research Laboratory launched its Experimental Fuel Cell (XFC) UAV, a folding-wing mini-drone called the Sea Robin, from the torpedo tube of the USS Providence, a Los Angeles-class fast-attack submarine.

When the Navy outlined its “Integrated Undersea Future Strategy” in 2011, it anticipated the need for more versatile “payload tubes” that could launch not only kinetic weapons but also alternative payloads such as the XFC or recoverable undersea vehicles. Soon afterward, it introduced the expanded Virginia Payload Module (VPM), which will ultimately triple the number of launch tubes on future generations of Virginia-class submarines.

The XFC brings the Navy a step closer to realizing the goals of the Upward Falling Payload program developed by the Defense Advanced Research Projects Agency (DARPA), which envisions a network of pre-positioned pods on the sea floor, awaiting commands to deploy assets (i.e., laser attack systems, surveillance sensor arrays, or flying or aquatic drones) and execute missions. These dormant outposts will presumably be linked by an undersea data network that would be connected, in turn, to surface, air, and space assets.

“To sustain its undersea advantage well into this century,” wrote Clark, “the U.S. Navy must accelerate innovation in undersea warfare by reconsidering the role of manned submarines and exploiting emerging technologies to field a new ‘family of undersea systems.’”

This new role for the manned submarine – a kind of undersea mothership for the tools and technologies that will engage adversaries up close – may be difficult, at first, for traditional submariners to accept. But the sustained mutual deterrence of the Cold War is evidence that the world’s military leaders have learned the lessons their predecessors often failed to grasp during the world wars: Naval warfare is in constant flux, and to underestimate the disruptive potential of the submarine, regardless of the nature of the conflict, is to risk all.