All matter in a black hole is squeezed into a region of infinitely small volume, called the central singularity. The event horizon is an imaginary sphere that
measures how close to the singularity you can safely get. Once you have passed the event horizon, it becomes impossible to escape: you will be drawn in by the
black hole's gravitational pull and squashed into the singularity.
The size of the event horizon (called the Schwarzschild radius, after the German physicist who discovered it while fighting in the first World War) is
proportional to the mass of the black hole. Astronomers have found black holes with event horizons ranging from 6 miles to the size of our solar system. But in
principle, black holes can exist with even smaller or larger horizons. By comparison, the Schwarzschild radius of the Earth is about the size of a marble. This
is how much you would have to compress the Earth to turn it into a black hole. A black hole doesn't have to be very massive, but it does need to be very
Some black holes spin around an axis, and their situation is more complicated. The surrounding space is then dragged around, creating a cosmic whirlpool.
The singularity is an infinitely thin ring instead of a point. The event horizon is composed of two, instead of one, imaginary spheres. And there is a region
called the ergosphere, bounded by the static limit, where you are forced to rotate in the same sense as the black hole although you can still escape.
In a spinning black hole, a central ring singularity is surrounded by two event horizons, the ergosphere and the static limit.