The diameter of this pin is the same as a black hole with the mass of Earth.
If you compress any amount of mass, it becomes denser. If you compress it enough, it will become so dense that light can no longer escape it—this size is the object’s Schwarzschild radius. An object that is compressed smaller than this radius is a black hole.
If you compressed all of the mass of Earth into the diameter of this pin, it would be so incredibly dense that you would not even be able to see it.
Every object has a Schwarzschild radius proportional to its density: the Sun's is 3km, Jupiter’s is 2.62m, and a hamburger’s is smaller than a single atom. Karl Schwarzschild derived this property in 1915 using Einstein’s recent field equations.