Chak "rock" is the airless fourth moon of a gas giant planet in a system so undistinguished it is not even given a name in Romulan star atlases. It is simply known by its catalog number. The planet could not be of any possible importance to anyone, which is why it was selected as the site of a top secret Romulan military installation. The surface station, supposedly a communications relay point, has an official personnel complement of six persons. Unofficially, 200 carefully selected Romulan officers and enlisted personnel (officially listed as "killed in action" over the years) are permanent residents underground, working on advanced military research projects, including an improved plasma weapon, a starcracker bomb (that, if constructed, would be as big as a heavy cruiser and could induce a nova-like explosion), and the development of a super-warp engine (similar to the UFP transwarp drive, but even less stable). The latter project is the only one really close to completion. (Best estimates say it will be ready to test within six years.)
The station is extremely expensive to maintain, because all important support functions must be concealed, and supply vessels must come and go under cloaking devices. Station security is perhaps the best in the galaxy. Even so, the project may be abandoned, as the Romulans simply cannot afford to maintain it much longer.