The Io Facility was a top secret Starfleet Section 31 construction hangar located in orbit of the Jovian moon Io, at coordinates 126.96.36.199 in the Sol system. Shuttlecrafts could enter the interior bay of the large enclosed drydock when panel sections on the side of the dock were rotated and moved out of the way. These crafts ferried parts for a starship to at least one of twelve loading docks.
The Dreadnought-class starship USS Vengeance was constructed at the facility and launched in 2259, under the command of Admiral Alexander Marcus. Before the launch, Montgomery Scott piloted a shuttlecraft into the interior of the facility, after Captain James T. Kirk asked him to investigate the hangar's spatial coordinates, supplied by Khan Noonien Singh. Scott was able to stow away aboard the USS Vengeance before it departed from the hangar. (Star Trek Into Darkness)
Background information Edit
The facility was only referred to as a "construction hangar" in the film. The name "Io Facility" comes from a non-canon Starfleet Top Secret Security Review. Khan, in the guise of John Harrison, had been stationed there.  A concept illustration of the facility can be viewed here.
The black-colored hull of the construction facility proved difficult to light in space. Explained Co-Visual Effects Supervisor Pat Tubach, "Working with our art director Yanick Dusseault we came up with some concepts where we would pass him something and say what do you think we should be doing, how should we light the black hangar, where do we put the light? We know where the sun is, but that doesn’t really provide a very interesting frame, and you would get back images that were almost like models or miniatures lit in space, and you start to realize that’s what looks cinematic and good and meets with your expectations about the way you should be lighting something.” 
According to the novelization of Star Trek Into Darkness, the facility's proximity to Jupiter's magnetosphere and Io's plasma torus caused spectrum distortions and upper hybrid resonance instabilities. These phenomena affected scanning and allowed Scotty's shuttle to remain undetected by the facility.