Jonathan Oliver is chasing down an intruder on Iron Mountain in Lake Oswego when an unknown foe traps him inside a cave yielding clues to Oregon’s distant past. He and his grandfather present his finds to a university expert, who pronounces them to be genuine. Then Jon and his father stumble upon a remarkable spinoff from their anti-gravity inventions, a discovery that nearly costs Jon his life. Together, they incorporate this breakthrough into a revolutionary submersible design. At the invitation of Dr. Graham MacKenzie, the Gyrosensors team travels to one of Scotland’s wildest and remotest regions—and its deepest lake—to test their secret prototype. While staying at Dr. MacKenzie’s retreat center, they meet a pretty but mysterious Frenchwoman going under an assumed name. Across the loch, the crusty, reclusive caretaker of a Victorian-era hunting lodge is hiding staggering secrets of his own. During the prototype's construction and sea trials, Jon and his family confront enemies on land, in the air and under the water. In the end, Jon rekindles a love from the ashes of his youth and uncovers an ancient, holy treasure-hoard. Illustrated by Becky Miller.