When I saw this first article, it reminded me of when I was a kid. We have a cave that I remember crawling into and I must say as an adult, they are still a bit intriguing. That said, here are a couple of articles on where to find a hole.
The other article is actually entitled “New Cave Photos: Inside Gabon’s Longest Mapped Cave” and over at National Geographic with photos by Trevor Frost. Wow!
Into the Darkness!