From the back cover:
The Known World may be Hollow but it isn’t Empty!
From Niels Klim’s Underground Travels to A Journey to the Center of the Earth, readers have been fascinated with the idea of a secret world beneath our feet. For the inhabitants of Mystara, the Hollow World is no fable!
Journey once again to the Land of the Red Sun, where noon never fades, and time is measured by floating continents circling high amidst the clouds... a land where Mystara’s past, and the secrets of the Immortals, live eternal.