The Pluto Vault is a large base taking up the entirety of Pluto and Charon. Bearing similarities to old Earth installations such as the Norwegian Seed Vault and Area 51, it is suspected to store some of the most sensitive documents ever created by Human civilization. It was supposedly formed at February 25th, 2990 CE.
The military of the Confederacy of Borealis has never commented about why the planet was blockaded like it is. Various leaks, however, have painted a fairly decent picture of what goes on inside of the planet, and one of the more famous incidents was called "Project Umbra".
History
Towards the end of the Second Age, the UFSS was severely weathered by countless wars. Desperate, the UFSS began to increase weapons development ten-fold to assist the war effort. Suspicions circulated that the UFSS had removed laws preventing the development of overly inhumane weapons in order to help the war effort. The UFSS denied these claims, stating them as "preposterous" and "unpatriotic.".
At the very end of the second age, the UFSS signed the Confederacy of Borealis, resigning leaderships of the star system Sol to a government represented by the surrounding planets. Several years after the signing, Pluto, previously a small, desolate world, was cordoned off, allowing large scale construction upon the surface. When questioned about the nature of the establishments, the government stated it was a "prime source of water-ice," and the construction was of a "fuel depot for future settlements,"
Suspicions
During the second age and the UFSS's dramatic increase in weapon technologies, rumors soon spread that laws preventing the construction of inhumane or torturous weapons had been temporarily removed, permitting biochemical warfare, advanced nuclear weapons, and weaponized planetary collisions, even permitting the removal of atmospheres from planets. Those of this belief failed to produce any evidence for or against the theory. Soon after, the public's focus was directed towards the signing of The Confederacy of Borealis, and the thought was largely, although not entirely forgotten.
Several years later, the topic returned when the system began to construct installations on the surface of the planet. Some theorized that, to strengthen trust between worlds, the plans for the now highly controversial plans were hidden in some form of underground bunker to prevent trial from exterior governments. There is no known evidence that supports this theory.
Today, the Pluto Vault is it's own form of myth. The secrecy of the location of the Vault has intrigued many people to the point where it is used in common language today, usually to express eccentricity or strangeness, usually in a humorous fashion.