Reportedly the prototype/initial base will support 12 men. Then it will either be expanded or replaced so that it can support 33 men. It wil lbe nuclear powered and, contrary to earlier reports it will be a fixed seafloor habitat rather than a mobile submarine.
The official story is that the base is meant to facilitate deep sea mining. However even in very rough surface weather, deep sea mining robots can be operated via tether from a conventional sub loitering just 200 feet down.
There is no application I know of other than military which would justify such an expense. But then, 9,800 feet is many times deeper than the crush depth of nuclear subs, which is ~1,600 feet. A base at this depth would only be able to service much smaller craft, like deep diving research submersibles.
What are they building this for? What’s down there that would be worth so much?