Usually, people discuss the “exterior” and “interior” of a black hole Snowboarding sit on Helicopter Christmas shirt. These regions are separated by the event horizon. If you cross from the exterior to the interior, you can never come back. A Schwarzschild black hole (one that has no spin) is particularly simple. What lies in your future, should you cross into the interior, is the “singularity” – a region where the curvature blows up in a certain way. You can’t avoid being stretched like spaghetti and crushed as you go forward in time. In a finite amount of time, you’ll be shredded and crushed and that’s it. The Schwarzschild solution is not considered to be particularly realistic. It’s hard to imagine forming a black hole with absolutely no spin. Once you move away from the non-spinning case, things get much more interesting. The spinning black hole solution is called a Kerr solution, and it has a very interesting property. There are singularities that are only coordinate singularities (like the fact that at the North pole, you can’t go farther North). One’s path can actually be extended through those singularities, and what lies on the other side looks like another copy of the same spacetime. Assuming the black hole was large enough (so the curvature was low enough that you wouldn’t get shredded and crushed), you could go through to another space.
First of all, the black hole solutions are “eternal” solutions – they have always existed and always will. They are not realistic in the sense that real astrophysical black holes form from the collapse of matter. That means that their “causal structure” is wrong. Overly critical. As Ph.D., we are taught to analyze and break things down. That is good for research but not so good for real life. I have seen friends argue over simple things just because they could not take a joke but instead had to systematically break it down. In spousal relationships, I have heard a Ph.D. student tell his wife why he is absolutely right by breaking down the argument and doing something like a full presentation. That did not go well for him. Most schools teach their Ph.D. students in certain ways. This creates a lot of bias. For instance, market efficiency people will literally think that market irrationality people are not smart and even crazy. Because of training, Ph.D. students may have certain strong biases. Despite all these negative issues, I still enjoyed my Ph.D. I learned a lot. I learned how little I know. I became more humble. I also learned how to question things and do my best to answer interesting questions.