-Russian President Vladimir Putin recently said Russia was developing a nuclear-powered torpedo that could detonate a "massive" nuclear weapon
-Such a device might create a 300-foot tsunami if exploded in the right location and could rain long-lasting radioactive fallout on a coastal target.
-Experts have described the hypothetical weapon as a "doomsday" device, saying it could spread unprecedented and long-lived radioactive fallout.
-But one researcher said such a weapon would be "stupid," as it'd greatly limit its explosive power and radioactive fallout compared with an airburst.
During Russian President Vladimir Putin's address to the Federal Assembly on March 1, he described a plethora of nuclear weapons he said Russia was developing.
One of these proposed weapons, an autonomous submarine, stood out among the depictions of falling warheads and nuclear-powered cruise missiles.
According to a Kremlin translation (PDF) of Putin's remarks, he said the autonomous drone would quietly travel to "great depths," move faster than a submarine or boat, "have hardly any vulnerabilities for the enemy to exploit," and "carry massive nuclear ordnance."
"It is really fantastic, he said, adding: "There is simply nothing in the world capable of withstanding them."
He also said Russia finished testing a nuclear-powered engine for the drones in December.
"Unmanned underwater vehicles can carry either conventional or nuclear warheads, which enables them to engage various targets, including aircraft groups, coastal fortifications, and infrastructure," he said.
The mushroom cloud caused by the Soviet Union's Tsar Bomba 57-megaton nuclear test.Minatom/Wikiepdia
Putin did not refer to the device by name in his speech, but it appears to be the Oceanic Multipurpose System Status-6, also known as Kanyon or Putin's "doomsday" machine.
The Russian government reportedly leaked a diagram of such a weapon in 2015 that suggested it would carry a 50-megaton nuclear bomb about as powerful as Tsar Bomba, the largest nuclear device ever detonated.
Nuclear physicists say such a weapon could cause a local tsunami, though they question its purpose and effectiveness, given the far more terrible destruction that nukes can inflict when detonated aboveground.
Why Putin's 'doomsday' device could be terrifying
A nuclear weapon detonated below the ocean's surface could cause great devastation.
The US's underwater nuclear tests of the 1940s and '50s, including operations Crossroads Baker and Hardtack I Wahoo, demonstrated why.
These underwater fireballs were roughly as energetic as the bombs dropped on Hiroshima or Nagasaki in August 1945. In the tests, they burst through the surface, ejecting pillars of seawater more than a mile high while rippling out powerful shockwaves.
Some warships staged near the explosions were vaporized. Others were tossed like toys in a bathtub and sank, while a few sustained cracked hulls and crippled engines. Notably, the explosions roughly doubled the height of waves to nearby islands, flooding inland areas.