[Bullet Magnet]

In the Swartzchild solution, which describes the spacetime outside of a sphere of matter and also a black hold in a matter-free universe, r is the distance from the center of the sphere/black hole.

ds² = −(1 − 2M/r) dt² + 1/(1 − 2M/r) dr² + r² dΩ2.

As r gets smaller the equation gets closer to infinity. At r=0 it reaches it. Or not, since it's usually considered to be undefined. But in a black hole, r=0 is the singularity. That is where the laws of physics as we understand them break down, because our only way of describing it returns as a divide by zero error.