From the Inside Out: Is the Hollow Habitable?

Umberto Eco: There are two Hollow Earth theories. According to the first one we live on the crust, but there is another world on the inside where lies, some say, the realm of Agartha, the home of the King of the World (see, for example, the fantasies of French philosopher René Guénon). (IHT, "Outlandish Theories: King of the Hollow World")

Image: Edmund Halley holding a drawing of his Hollow Earth