Basically, when you try to combine them, the equations break, they give infinities, they just don't produce answers or calculations that you can use that you can work with.
Our best theory of space and time, Einstein's theory of general relativity, says that space and time become infinitely curved, that the centre of the hole becomes infinitely dense.
You could, theoretically, go all the way down to the molecular level for your measurement unit but if you do that, the length of the coast seems to approach infinity.