Some schools have hidden lockers.Lincoln Elementary has a hidden world.When sixth-grader Theo Ellis spins the old classroom globe one rainy afternoon, he doesn't expect it to pop open — or to find a dusty, hand-drawn map tucked inside. Even stranger? The map doesn't lead to anywhere above ground. It points below.Together with his best friends, book-smart Zoe and daredevil Marco, Theo sets out to follow the clues. But what starts as a harmless after-school adventure q...