the Relic of Hollow Earth: Relics Forgotten World series

A Viking longship buried in a peat bog should contain bones, iron, and forgotten grave goods. Instead, Dr. Freya Holm discovers a hidden map carved beneath its keel-and directions to a door that was never meant to be opened. The ancient trail leads Freya and her expedition into a forbidden mountain range, where a colossal stone entrance descends toward an impossible civilization. Far below the surface, thousands of blue lanterns illuminate a city whose people have never seen the sun. But the city is not alone. Something ancient sleeps beneath its temple. The inhabitants call it the Keeper, and for generations they have fed it with offerings of "living light." Now the surface explorers have broken an old agreement, awakened a buried hunger, and opened a path that their Viking predecessors sacrificed everything to close. With their lights failing and the mountain beginning to move, Freya must uncover the true purpose of the longship map before the darkness below learns how to reach the world above. The Relic of the Hollow Earth is a gripping illustrated archaeological fantasy thriller filled with ancient mysteries, subterranean civilizations, supernatural danger, and cinematic adventure.