Is this part of the secret content, or something else?
Metal Gear Solid 5: The Phantom Pain has been so popular since it was launched on September 1 that it shipped more than 3 million copies in the first few days. With that many people out there looking for “secret content” it’s no wonder that one user actually found a glitch that allows people to explore an area of the map that was previously unreachable. The part of the game that was previously not accessible is apparently located in the Serak Power Plant and is the place where Metal Gear Sahelanthropus is apparently being kept.
A user on Reddit takes you from there and provides the rest of the secret in order to get into this part of the game:
This is one of two known rather large glitches. The other is a Metal Gear Solid V The Phantom Pain glitch that keeps users from actually attaining a 100% completion rate.