Question

What's at the bottom of a black hole?

Answer 1

First of all, black holes are three-dimensional objects: there's no bottom, in the same way that there's no bottom of the earth; there's only a center.
The "surface" of a black hole, or more precisely, its event horizon, is the boundary beyond which nothing, not even light, can escape from the black hole's gravitational pull. Actually, in the reference frame of an outside observer, due to time dilation, an effect of general relativity, as objects approach the event horizon, the increased gravity they experience causes them to slow down so that they asymptotically approaching the horizon, never actually passing through.
For somebody standing outside a black hole, nothing ever passes through the event horizon, so in that sense, all of the mass of a black hole is present right on the surface. This means the inside of a black hole is actually empty, to all outside observers!..................................

Answer 2

If you use Einstein's theories to determine what occurs at the bottom of a black hole, you'll calculate a spot that is infinitely dense and infinitely small: a hypothetical concept called a singularity. But infinities aren't typically found in nature.