When it dies, its gravity becomes so strong powerful it collapses in on itself. Since the center is iron, the mass hits it and bounces off. That creates a supernova explosion, the most powerful thing in the universe. Then the debris from the explosion implode and the gravity becomes so strong, it will suck anything in. That is a black hole.
Nobody knows for sure, but scientists made some good hypotheses.
If you were going in feet first, the gravity would be stronger on your feet. Then you will be stretched 20 to 30 feet, and then you will snap in half. Time stops in a black hole. When you enter a black hole, you will see the future. You may be warped to another universe.