Play in the arcade now.
Chances are that you've finished Peace Walker's main story by now. And, at the end, you are told by Kaz that you got some mail. No return address, but it was signed by someone named Eva. Kaz never knew about Eva, only Snake did. Well, in the box are some tapes explaining some huge secrets about the Boss and why America sent her on the mission of defecting to the Soviet Union and much more.
To access these secrets, you must have first obtained the mail from Kaz. Then, go to Mission Selector from the Mother Base. Select any mission you want and go to Briefing Files. Then, press the R button to access the file library. Scroll down to the bottom and select Data Files. From there, you have eight tapes from Eva.
The first tape was the tape Eva left for Snake in MGS3. Yes, it did self destruct, but somehow it is in Snake's files. You may also remember that this is the tape that was playing in the background before and after Snake is crowned with the title of Big Boss. But that's not what we care about, we've heard that before.
As for the next seven tapes, click "Message from EVA" and listen to all seven tapes in chronological order. You will want to hear what she has to say. If you have a PSP and Peace Walker, do not read the next paragraph, it will contain spoilers from the tapes. If you wish to check out the last paragraph, it has no spoilers. But do this at your own risk, your eyes may wander.
SPOILER ALERT after the image!
Okay, the tapes basically say that The Boss was the eighth astronaut of Mercury Seven. However, all around America, pictures of Mercury Seven edited out The Boss. It was a big secret that she was one of the astronauts. The secret was so big that the team was known as the "Mercury Seven."
Now you're thinking "who cares, she was an astronaut, big whoop." Well, don't you want to know why it was such a secret? America had to catch up to other countries in the Space Race. After all, if a satellite could be sent into space, so could a rocket.
So, the American Government gave The Boss the mission of defecting to the Soviet Union so that she could steal some intel to help Americans reach greater success in the Space Race.
That's the most important part of the tapes if you ask me, but they go on and on long enough to have many additional details. I suggest you check them out. I personally didn't come across these tapes until today because I have been so busy with Main Ops and Extra Ops. You might be in the same situation.