Wikipedia and some other web sites mention that: At the center of a black hole lies the singularity, where matter is crushed to infinite density, the pull of gravity is infinitely strong, and spacetime has infinite curvature. This means that a black hole's mass becomes entirely compressed into a region with zero volume. This zero-volume, infinitely dense region at the center of a black hole is called a gravitational singularity. My question is, though it is true that the center of a black hole has an even higher density than other region inside the black hole, it does not necessarily mean that it must be a singularity, with infinite density and zero-volumn. I think the total amount of mass inside a black hole is finite, the volume of the center region is also finite, so the density is also finite, even though it can be arbitrarily high. Another question is if singularity does exist in the center of a black hole, how is it formed? Is it formed gradually over time as mass moves closer and closer to the center, or is it formed instantly as soon as the black hole is formed? Thanks!