Key point: The Navy must learn from its past experiences and never take the survivability of carriers for granted.

A photo depicting an American nuclear-powered submarine poking its periscope above the waves—within shooting distance of a British aircraft carrier during a war game—is a useful reminder of one of the most important truths of naval warfare.

For every sailor who’s not in a submarine, submarines are real scary.

Stealthy and heavily-armed, subs are by far the most powerful naval vessels in the world for full-scale warfare—and arguably the best way to sink those more obvious icons of naval power, aircraft carriers.

The public may not fully appreciate submarines’ lopsided combat advantage, but the world’s leading navies sure do. Today Chinese, Russian and American submarines, among others, are busy sneaking up on, tracking and practicing sinking rival fleets’ flattops.

The provocative photo, see here, depicts the masts of the U.S. Navy attack submarine USS Dallas near the carrier HMS Illustrious during a naval exercise in the Gulf of Oman on Oct. 3, 2013. Six warships including Dallas and Illustrious conducted an anti-submarine-warfare exercise that saw Dallas stalking Illustrious while British and American surface warships and helicopters attempted to locate and “sink” the undersea vessel.

Neither navy has published the results of the exercise, so it’s not clear whether Dallas got close enough in the course of the war game to simulate firing Mark-48 torpedoes at the flattop, which at 22,000 tons displacement is one of the largest ships in Royal Navy service.

But there are good reasons to assume the 7,000-ton Dallas did succeed in pretend-sinking Illustrious. In 2007 HMCS Corner Brook, a diesel-electric submarine of the Canadian navy, sneaked up on Illustrious during an exercise in the Atlantic.

To prove they could have sunk the carrier, Corner Brook’s crew snapped a photo through the periscope—and the Canadian navy helpfully published it. “The picture represents hard evidence that the submarine was well within attack parameters and would have been successful in an attack,” boasted Cmdr. Luc Cassivi, commander of the Canadian submarine division.

Corner Brook, a former British submarine displacing only 2,400 tons, is no more capable than Dallas—and probably much less so once crew training is taken into account. American submariners spend far more time at sea than their Canadian counterparts.

Dallas and Corner Brook scored their simulated carrier kills against allied warships in the context of a scripted exercise. But many other close encounters between subs and flattops have occurred between rival nations deep at sea, in a usually bloodless duel that is nevertheless deadly serious.

To prepare its submarines to hunt and sink American aircraft carriers in some future World War III, during the Cold War the Soviet navy ordered its hundreds of sub captains to get as close as possible to U.S. flattops … and stay there. The U.S. Navy routinely surrounds its multi-billion-dollar carriers with escorts including surface ships and submarines, but the defensive screen is not impenetrable.

In 1974 a Soviet Il-38 patrol plane spotted what was later described as the carrier USS Nimitz and its escorts off the U.S. East Coast. The ship’s identity is in doubt, as in 1974 the brand-new Nimitz was in the water at a Virginia shipyard and still being worked on.

Whichever carrier it was, Soviet commanders instructed an attack submarine to track the flattop and its escorts. “Three days we [followed] Nimitz [sic],” navigator Pavel Borodulkin told Tom Briggs, an American who visited Russia decades later.

Borodulkin implied that the sub spent much of the time at a depth of 120 feet. As for being detected … “We did not worry,” Borodulkin said, explaining that American sonar was not optimized for detecting a target moving on the same course and speed as the vessel doing the searching.

“Our stealth was high,” Borodulkin said. To prove his claims, the navigator gave Briggs the above blurry photo of a flattop, snapped through the Soviet sub’s periscope.

That wasn’t the only NATO carrier the Soviets tailed. In 1984 a Victor-class Soviet submarine played cat and mouse with the flattop USS Kitty Hawk off the Korean Peninsula. The Americans lost track of the Victor and, in the dead of night, the 80,000-ton carrier actually collided with the 5,000-ton sub.

“I felt the ship shudder violently and, going to the starboard side, I could see two periscopes and the upper part of a submarine moving away,” Kitty Hawk Capt. Dave Rogers told The Sydney Morning Herald. A Japanese patrol plane later spotted the apparently damaged Victor limping away at three knots.

In November the same year Illustrious, then a young vessel, passed within 500 yards of a Soviet Tango-class submarine during a Royal Navy exercise off the Scottish coast, according to The Robesonian newspaper.

When the Soviets introduced their own small aircraft carriers in the mid-1970s, British and American subs no doubt watched them as closely as Soviet undersea boats followed NATO flattops. But there were no public accounts of Western subs getting caught doing so until 2007, when a Russian newspaper reported that warships escorting the carrier Admiral Kuznetsov in the Atlantic pursued an unspecified submarine for half an hour.

The snooping sub reportedly got away by deploying self-propelled decoys.

After the Soviet Union’s collapse in 1991, the Russian submarine force shrank considerably and, for a few years at least, was much less aggressive. The Russian carrier fleet declined to a single vessel, the Admiral Kuznetsov.

American attention gradually shifted east to the Pacific, where in the early 2000s China had launched a massive naval rearmament program that included refurbishing a former Soviet carrier, a sister ship of the Admiral Kuznetsov that was renamed Liaoning in Chinese service.

In addition to their new flattop, the Chinese built several new submarines per year on average, soon boasting a fleet of some 60 undersea boats—about as numerous as American subs.

Not nearly as large, advanced or active as U.S. subs, the Chinese boats were at a huge disadvantage. Beijing’s subs struggled to gather intelligence and develop wartime tactics. They enjoyed at least one dramatic success in October 2006, when a Chinese Song-class diesel-electric attack submarine quietly surfaced within nine miles of Kitty Hawk in the waters between Japan and Taiwan.

The Song-class vessel, displacing 2,200 tons, was close enough to hit the Kitty Hawk with a torpedo. None of the carrier’s roughly dozen escorting warships detected the Song until it breached the surface. American officers were flabbergasted.

“This could well have escalated into something that was very unforeseen,” said Adm. Bill Fallon, then commander of U.S. Pacific forces.

But it’s apparent that China is more scared of American submarines than the Americans are scared of Chinese boats. In 2012 Liaoning was finally ready to set sail from the Dalian shipyard. As Beijing’s only carrier facing a fleet of 10 American flattops, Liaoning was widely expected to stage from China’s most modern naval base on Hainan Island in the south, near Taiwan and Vietnam.

Instead Beijing announced the 70,000-ton carrier would be heading north to Qingdao. The apparent reason was that the area around Qingdao was already home to a squadron of Song-class submarines plus Type 091 nuclear subs. Those vessels are the best defense China possesses against the American and Japanese subs that will undoubtedly hound Liaoning every time she leaves port, practicing to sink the carrier in the event of war.

Doing, in other words, what submarines do best.

This article first appeared in 2016.

Image: Reuters.