It's one of the more unique, more capable, and more distinctive vessels in Russia's naval fleet. And it now appears to hold the distinction of being the latest Russian naval ship to suffer a catastrophic -- and, so far, unexplained -- incident.
Russia's Defense Ministry said on July 2 that a submarine operated by its main naval research-and-development unit had suffered a fire on board, killing 14 sailors. The ship was reportedly operating in Russia's territorial waters in the Barents Sea.
The ministry did not identify the ship involved in the July 1 incident, describing it only as "a research submersible vehicle designated for studying the seafloor...in the interests of the Russian Navy."
Multiple Russian media outlets, however, reported it was a submarine known as the AS-12; another outlet pointed to a vessel with a similar design, the AS-31.
Nicknamed Losharik, after a Soviet-era cartoon horse made up of balloon-like spheres, both submarines are nuclear-powered vessels that have been the subject of speculation and rumor among Western naval analysts and military experts for years, curious about its design and capabilities.
According to one respected naval blogger, the AS-12 is a deep-diving special missions ship, operated by the Russian Navy's primary research unit, the Main Directorate Deep Sea Research (GUGI).
Nuclear-powered, with a crew of up to 25, the ship has been in service since the early 2000s and is believed to be able to dive as deep as 6,100 meters.
It's designed in a way that allows it to dive deeper than normal Russian attack or ballistic-missile submarines. It features a series of orbs contained within a traditional-looking submarine hull, thus giving rise to its cartoon nickname.
The submarine is also designed to be carried, or ferried, under the belly of a larger submarine, allowing greater secrecy and shrouding from sonar or other surveillance.
Western military analysts and at least one Russian naval blogger said the ship that most often ferries the AS-12 is the BS-136 Orenburg, a modified Delta III ballistic-missile submarine.
Though military spending has started leveling off, Russia President Vladimir Putin has poured tens of billions of dollars into upgrading his country's armed forces and increased the number of operations across the different branches in recent years.
The navy is no exception, particularly with the increased Russian military presence in Syria and the eastern Mediterranean, says Jeffrey Edmonds, a former National Security Council staffer and research scholar at the Washington-based Center for Naval Analyses.
"A higher operations tempo increases the chances of an accident," he tells RFE/RL. "However, it's too early to say what caused the fire and whether or not it was human or mechanical error. The Losharik incident will likely have a deep operational impact on the Directorate for Deep Sea Research, given how advanced and relatively few these submarines are."
For the wider public, the shadowy world of undersea surveillance, and even sabotage, has regained attention over the past two years with the spotting of Russian military research vessels lurking off U.S. coastlines.
U.S. intelligence and military officials have publicly voiced concerns that Russian forces might be developing new, secretive ways to tap -- or even cut -- undersea fiber-optic cables that carry transatlantic Internet traffic.
Others have pointed to new Russian efforts to go after the network of undersea acoustic arrays that the United States and NATO have deployed for years to track submarines, or even classified naval cables.
GUGI, the navy’s leading experimental research division, has been behind several eye-catching weapons systems in recent years, including a nuclear-powered torpedo.
It's also responsible for a research vessel known as the Yantar, which was launched in 2015 with the ability to carry two manned submersibles and a remotely operated underwater vehicle.
In 2017, Russia's official government newspaper, Rossiiskaya gazeta, boasted of the surveillance capabilities of the ship, which, like the AS-12, calls the Kola Peninsula port of Severomorsk its home.
"Yantar has devices aboard intended for deep-sea tracking, as well as equipment for connecting to top-secret communications cables," the paper wrote.
In 2018, the United States went so far as to accuse Moscow of "tracking undersea communications cables" and imposed economic measures on the Russian company that was allegedly providing underwater diving equipment to Russia's Federal Security Service.
On July 2, there were conflicting reports about what may have sparked the fire, whether the dead sailors may have asphyxiated due to noxious fumes or for other reasons, and whether the fire occurred on the submarine itself or on another submersible launched by the AS-12.
For many Russians, the incident has echoes of one of the most searing moments of Putin’s presidency: the 2000 sinking of the submarine Kursk, which killed 118 sailors. Putin and military officials were excoriated in the immediate aftermath for lying about the rescue operation.
"The Russians are investing a lot of energy in developing new approaches to undersea warfare, but everything that I have seen...seems to indicate that certain legacies of the Soviet era, especially overly centralized design control and corruption, remain endemic," says Don Thieme, a retired U.S. Marine officer and now a professor at the Center for Naval Warfare Studies at the U.S. Naval War College.
Despite lingering problems with design and corruption, Thieme -- who clarifies that his views are separate from those of the U.S. Navy or the Defense Department -- says Russian military thinkers are still formidable.
"One should never underestimate the Russian capability to be innovative in their design processes and how they think about the maritime domain from their perspective," he tells RFE/RL.
"No matter what the cause, it is a sad day for the Russian submarine force and the Russian Navy who will, once again, have to do a lot of soul-searching and answer some painful questions," he adds.