The fourth Yasen-class submarine, the Krasnoyarsk, has passed critical tests of its structures and pressure hull, to Russia’s TASS news agency. It’s supposed to be the most quiet nuclear-powered attack sub ever to enter the Russian fleet.

As TASS reports, the Krasnoyarsk’s pressure hull withstood hydraulics tests and that further work is being planned for installation and assembly. The hull is built with low magnetic steel to help it avoid detection, to The Diplomat. Keep in mind that Magnetic Anomaly Detectors were a large part of Cold War .

Russia has five Yasen-class subs under construction at the Sevmash Shipyard, in Severodvinsk, Russia. The only one in service at the moment is the K-329 Severodvinsk. That boat is 390 feet long and 13,800 tons; . It has “The vessel has eight torpedo tubes, four of which are 650mm tubes while the rest are 533mm tubes,” according to The National Interest.

As far as speed, it can travel as fast as 35 knots, and is considered the in Russia’s fleet. Russia has had much faster subs back in the day, that could supposedly top 44 knots underwater, though that almost certainly wasn’t very quiet.

That said, the Yasen class subs U.S. Navy officials over fears the Russians are encroaching on their decades-long underwater dominance. Rear Admiral Dave Johnson, Naval Sea Systems Command’s program executive officer, was so impressed with the Severodvinsk that he had a model of the sub for his office just so he could look at it every day.

But the Severodvinsk is not exactly a dominant competitor with America’s top subs when it comes to silence. It doesn’t move through the water with less noise than the U.S. Navy’s Seawolf- or Virginia-class submarines, Navy sources told The National Interest in March. The Virginia 15 officers and 117 enlisted men. As far as its own armament, the Virginia has space for twelve VLS tubes and four torpedo tubes, the latter of which can fire Mark 48 ADCAP torpedoes.

Though neither may be as quiet as subs with AIPs (air-independent propulsion system), that are supposed to be that they can skip detection. The U.S. Navy has none of these AIP subs, while China has as many as 15 of them. To get a feel of the technology, the U.S. Navy leased one of Sweden’s AIPs, HSwMS Gotland in 2005to see how well it would stack against its vessels.

As Foxtrot Alpha in 2014, the Navy didn’t fair so well:

By mid summer of 2005 the Gotland arrived in San Diego and war games immediately commenced. Apparently the Navy got more than they were bargaining for when it came to finding and engaging the stealthy little sub. The Gotland virtually “sunk” many US nuclear fast attack subs, destroyers frigates, cruisers and even made it into the ‘red zone’ beyond the last ring of anti-submarine defenses within a carrier strike group. Although it was rumored she got many simulated shots off on various US super-carriers, one large-scale training exercise in particular with the then brand new USS Ronald Reagan ended with the little sub making multiple attack runs on the super-carrier, before slithering away without ever being detected.

The lesson was clear: bigger and nuclear doesn’t always win.

As for the Yasen-class subs, they may not be a top competitor against the Navy’s submarine fleet at the moment, but the Russians are slowly getting the technology down. While the Severodvinsk may not be able to compete with the Seawolf or Virginia-class subs, the Krasnoyarsk may be the one that finally does.