It seems to me possible that as computer science advances and we get better and better at simulating things, that someday we will simulate an entire universe (in its own set of dimensions so it won't get in anyone's way -- of course the computer would generate the appearance of dimensions -- there would be no need to actually create them).

Given that this is a possibility, and given that anything possible must necessarily happen given time, and given that in any such simulation intelligences would in turn evolve who would in time begin to make their own simulations (simulations inside simulations etc.), it begins to get scary.

In fact, it would seem we are almost certainly, given the laws of probability, inside such a chain of simulations now.