Beliefs in mysterious underworlds are as old as humanity. But the idea that the earth has a hollow interior was first proposed as a scientific theory in 1691 by Sir Edmond Halley (of comet fame), who suggested that there might be life down there as well. Hollow Earth traces the surprising, marvelous, and just plain weird permutations his ideas have taken over the centuries. From science fiction to utopian societies and even religions, Hollow...