When I was younger, I had a CYOA that involved falling into the inside of the Earth. You discover that the Earth is held together by a "black sun" which turns out to be a black hole. Some of the potential endings include becoming king of the people living on the inside of the Earth and falling into the black sun. Does anyone have any idea what this is?
I actually found it! It's called Underground Kingdom by Edward Packard. This one is intense. I died a lot as a kid.
You are exploring the Underground Kingdom, a fantastic and strange world in the center of the earth. As you stand inside the largest cavern you've ever seen, you are amazed at the dazzling sight before your eyes.
If you decide to climb the Shining Mountains, turn to page 36. If you explore the Weightless Peaks, turn to page 85. If you go to the Hills of Diamonds, turn to page 88. Be careful! The Underground Kingdom can be dangerous. You might be attacked by the vicious Kota beast—or caught in the middle of the war between the Rakas and the Archpods, two barbarian tribes. You may not make it back alive!
What happens to you in the Underground Kingdom all depends on the choices you make. How does the story end? Only you can find out! and the best part is that you can keep reading and rereading until you've had not one, but many incredibly daring experiences!