A SOMEWHAT MORE ELABORATE type of parallel universe emerges from the theory of cosmological inflation. The idea is that our Level I multiverse--namely, our universe and contiguous regions of space--is a bubble embedded in an even vaster but mostly empty volume. Other bubbles exist out there, disconnected from ours. They nucleate like raindrops in a cloud. During nucleation, variations in quantum fields endow each bubble with properties that distinguish it from other bubbles.