What is a black hole?

What is a black hole? A black hole is a region of space that has so much mass concentrated in it that there is no way for a nearby object to escape its gravitational pull. If you threw the rock straight up in the air hard enough, you could make it escape the planet's gravity entirely. It would keep on rising forever. The speed with which you need to throw the rock in order that …

…that is, towards larger and larger values of t). Eventually, you're bound to hit the singularity at r = 0. You might try to avoid it by firing your rockets, but it's futile: no matter which direction you run, you can't avoid your future. Trying to avoid the center of a black hole once you've crossed the horizon is just like trying to avoid next Thursday. Incidentally, the name 'black hole' was invented by John Archibald Wheeler.