When a July 2015 undersea tremor triggered a rockslide between the islands of Saipan and Tinian in the Northern Marianas Islands, it cut the only fiber-optic cable connecting the archipelago to the global network. Air traffic control grounded flights, automated teller machines shut down, web and phone connections broke.

All the feared impacts of a cyber attack became real for the islanders. A Taiwan-based cable repair ship eventually restored the link, but that was a single break from one natural occurrence. How much more disruption could a deep-sea-faring nation cause its rivals through malicious intent?

Though often mentioned in passing, the fact that the overwhelming bulk of Internet activity travels along submarine cables fails to register with the public. High-flying satellites orbiting the crowded skies, continent-spanning microwave towers and million miles of old 20th Century copper phone wire all carry but a fraction of the Earth's Internet traffic compared with deep-sea fiber-optic cables.

All that buzz occurs in the dark cold parts of inner space and a few very quiet places on land. If you want to tap into that buzz, those quiet places where the sea cables make landfall—from the onshore facilities out to deep-water offshore—are your prized targets. The U.S. has developed exquisite abilities to access underwater things.

One of America's greatest techno-spy capers of the Cold War involved tapping Soviet Navy communications via a submarine cable in the 1970s and 1980s. Before IVY BELLS ended with its unmasking by Soviet spy Ronald Pelton, its clandestine aquanauts, spy sub and nuclear-powered "bug" made espionage history.

If, however, you wish to practice hybrid warfare—disruption and degradation with little overt engagement—then the ability to cut submarine cables at will and at depth gives you a very powerful weapon. Cut up undersea hydrophone networks and you deafen your adversary. Cut Internet cables and you have the ultimate denial-of-service cyber weapon.­

In the 1960s, America's advancing deep-sea capabilities alarmed the Soviets for good reason. If the Americans could locate sunken subs and retrieve satellites from the supposedly inviolable deeps, their underwater black ops directly threatened Soviet security.

Amidst a wave of 1960s deep-sea activity—including its saturation-diving SEALAB program, its search & recovery missions for USS Thresher, USS Scorpion, a lost H-bomb and a new submarine-rescue approach—Admiral Hyman Rickover got his dream sub built. Launched in 1968, the NR-1 was a marvel of American ingenuity—a nuclear-powered four-man minisub able to dive over 3,000ft and stay there for weeks.

During the later Cold War, the USSR developed its own very sophisticated underwater black ops capabilities. Targets included the US Navy's SOSUS underwater hydrophone system which ran along the Atlantic from Greenland to the United Kingdom at depths of up to 12,000ft. With its primacy in titanium metallurgy and skill in nuclear technology, the Soviet Union built several competitors to the NR-1.

The Russian Federation inherited these assets along with the Ministry of Defense branch (GUGI, or Main Directorate Deep Sea Research) that supervised those programs. The Russian deep-sea clandestine systems survived the collapse of the USSR and received funding even during the lean years during the 1990s. Even as the United States lost the NR-1's capabilities to retirement, Russia bolstered its deep-sea black activities.

Between August and October 2012, a Russian Arctic expedition mapped the seafloor of the Arctic Ocean to locate resources and establish maritime boundaries. A deep-sea "research" submarine placed a small Russian flag on the ocean floor some 13,200ft below the North Pole to mark Russia's claims.

The "research sub" was in fact one of Russia's secret deep-sea assets, the nuclear-powered minisub Losharik. A naval blogger pointed out that clues to Losharik's internal configuration lay in the vessel's name. "Losharik" is a cartoon horse made of linked spheres—some analysts suspect that the sub's interior consists of linked spherical titanium pressure hulls.

Losharik rides to its dive sites in the belly of a larger converted nuclear missile sub, the Orenburg. Both have been busy—the Orenburg recently surprised Norwegian scientists by popping up from beneath the sea ice after doing who-knows-what.

"[I]t was inexpedient to use GUGI equipment for scientific exploration and mineral extraction," said a Russian source. "On the other hand, they are the world experts in manned deep-water investigations and the training of their crews is on the same level as that of cosmonauts. If we don’t find a job for them, Russia is likely to lose this highly skilled workforce."

With Losharik, its sister sub Nelma and the two bathyscaphes Rus and Consul, the GUGI commands the largest manned deep-sea fleet in the world. Whether domestically produced or foreign imports, these assets can be quietly moved around within Russia itself. Combined with its growing submarine-rescue fleet and Naval Spetznaz outfits, Russia now fields a very powerful undersea hybrid warfare capability. Let's hope the US Navy is on the case.

Steve Weintz, a frequent contributor to many publications such as WarIsBoring, is a writer, filmmaker, artist, animator.