According to Popular Science, Bohai Shipbuilding Heavy Industrial Corporation is building the plant in Huludao, Liaoning Province. The place will have two parallel assembly lines. The gigantic hall reportedly is where China will begin construction on is latest attack submarine, the Type 095.
Despite having control over its Internet and press, the Chinese government has had enduring problems with military enthusiasts sneaking pictures of new ships and aircraft under development or construction. While some of this is useful to telegraph China's broader intentions and give a hint of its capabilities, the rest of it is considered a nuisance. Building an indoor submarine factory keeps sensitive subjects such as the hull shape of a submarine or even the propeller design a secret.
This does not necessarily mean China now has a submarine-building edge over the U.S.
The U.S. Navy commissions about two submarines a year, each of which takes about three years to build. That puts U.S. submarine construction at about six at a time, ahead of this factory's four. Of course, China builds submarines at other locations too, and may even build them a little faster.
Speed isn't everything. Even if China ends up capable of building more submarines per year than the United States can, there's the question of quality. The Type 095 is expected to be quieter that the second tranche of Los Angeles-class attack submarines that were built in the late 1980s, and Russian Victor III or Akula-class submarines built in the early 1980s.
In other words, China is approximately 30 years behind the United States in submarine quieting technology.
Chinese submarines in the 1980s were about as quiet as American submarines from the 1950s, while modern American submarines such as the Seawolf orVirginia-class boats are so quiet they're described as "quieter at 25 knots than the Los Angeles class at pierside." This is not a field where China has been able to close the gap.