The G Corperation's main Headquarters. Only the the most highly trained G Coperation soldiers are stationed here. It is likely that Kazuya Mishima would be found here.
This is a large base in which many G Corperation soldiers are stationed, this being their primary military base. Training of new G Corperation recruits also goes on here as well. Since there are so many soldiers here, I wouldn't advise entering without autorization.
This is an underground facility for all of G Corperations scientific and technological research, particularly focused geo-engineering. However, it's unlikely that it's the only research going on down there...