Black holes
Black holes are places where big stars have collapsed and effectively disappeared from the universe, except that there’s left behind a hole where the star used to be. So you have there a very strong gravitational field without any bottom. The black hole is the only place where space and time are really so mixed up that they behave in a totally different way. I mean, if you fall into a black hole, your space is converted into time and your time is converted into space.
It is sort of the ultimate relativity.
i likw this theorem x