A Romance of the Polar Pit, 1901

In Thyra, A Romance of the Polar Pit by Robert Ames Bennet, which was published in 1901, four explorers in a hot-air balloon drift and crash in an unexplored part of the Arctic. There they find a warm subsurface land with a Norse colony and fantastic creatures. As the land has survived unchanged for millions of years, it is inhabited by prehistoric mammals and dinosaurs, a bestial, ape-like race, and the Norsemen, descendants of Vikings who had discovered the land centuries before. The explorers save a young Norse girl, Thyra, and her brother from a giant cave bear and hostile Norsemen, the Thorlings. Thyra's clan, the Runefolk, accept the stranded explorers among them. At the invitation of the treacherous king of the Thorlings the explorers and a number of their Norse friends undertake a journey to his kingdom, where a gigantic idol of a prehistoric sea monster is worshipped by both the Thorlings and the beast-men. When they learn that they are to be sacrificed to the idol, the explorers destroy it, thereby unleashing a furious battle between the Norsemen and the ape-men. The Norsemen are victorious, the sea monster is destroyed, and the Norsemen are freed from its terrible influence.