Beyond Understanding — Richard L. Cederberg

Taken to the limits of their endurance the intrepid crew braves a subterranean world filled with dangers and bizarre wonders. When Thurids warriors are defeated the crew frees the Grettig family from captivity and all embark together on a vast Underground Sea inhabited by incredible creatures. Weeks later, from evidence on three parchments found in a mysterious wooden box, the crew splits up. Eight of the crew hike into the rugged Eastern Mountains in search of a fabled Lost City, while the others take both Subterranean Exploration Submersibles into the far northern territories to investigate the Himminglaeva Settlement. After a grueling trek through the mountains both parties somehow reunite at the Caldera Vale where, following a cataclysmal event in which one of the SES is destroyed, all are airlifted from the Realm of Ancients and flown up to Tatoosh Island, south of the Strait of Juan de Fuca, to re-board the newly renovated schooner Heimdall. After a deadly encounter with Mortiken soldiers they set sail north, along the western Canadian coastline, in their relentless quest to find the First Tribe of Rognvalds to present them with the legendary Tempest Sword and a fortune in blood rubies.