The Smoky God or, A Voyage to the Inner World

The Smoky God, or A Voyage Journey to the Inner Earth is a novel of 1908 by Willis George Emerson, which is presented as a true account of a Norwegian sailor named Olaf Jansen, and explains how Jansen's sloop sailed through an entrance to the Earth's interior at the North Pole. It is notable as an early story about an underground civilization.
For two years Jansen lived with the inhabitants of an underground network of colonies who, Emerson writes, were 12 feet tall and whose world was lit by a "smoky" central sun. Their capital city was said to be the original Garden of Eden. Later works by other authors, such as Agartha - Secrets of the Subterranean Cities, have identified the civilization Jansen encountered with Agartha (a mythical subterranean city), although Emerson did not use the name. (wikipedia.org)