A couple of times I've posted about an idea put forward by some freethinking physicists — namely, that our universe might be best understood as a virtual reality simulation.
What I didn't know is that there's a neat little Web site devoted to this idea, with some well-written essays that explain the concept in refreshingly nontechnical language.
It's called The Bottom Layer.
If this kind of ontological speculation appeals to you (as it does to me), check it out.