You're browsing the GameFAQs Message Boards as a guest. Sign Up for free (or Log In if you already have an account) to be able to post messages, change how messages are displayed, and view media in posts.
After you complete "Fly South" (mission where you go to south island and get wingsuit) you can no longer enter the cave. If you're past that, you need to start a new game and get to the point where you save Liza. That's the first opportunity to enter.