When siblings Lirien and Tivian stumble upon a mysterious atlas in their school library, they unlock a secret network of underground passages linking the world's most iconic landscapes and cultures. From the moonlit catacombs of Paris to the star-studded camps of the Sahara, from the Amazon's underwater opera house to Inuit villages beneath Greenland's ice, each journey challenges them to solve geography puzzles, meet new friends, and collect knowledge. As they sketch botanical specimens and map each hidden route, their atlas blossoms with vibrant illustrations and cultural anecdotes. Ultimately, they learn that true connection lies beneath borders—in shared soil, stories, and the spirit of exploration.