The thing to remember is that space isn't flat. Gravity warps it, like a bowling ball warps a flat bed sheet, but in 3 (technically, 4) dimensions. A black hole is a warping where instead of a gradual dip in the sheet, there is a cone that plummets down seemingly infinitely. But the principle of relativity means we don't really know what's on the other side; just that anything that goes over the edge of the quasi-spheroid line (the event horizon) can't ever go fast enough to come back.