The Secret City — Chris Archer

Spies. Underground tunnels. Old maps. New enemies. Pirate oaths. Hidden treasure. Endless danger.
This series has real boy appeal. It has a fast pace, adventure, international intrigue, and more.

George is fascinated by his family's legacy, the stories that link his ancestors to the infamous pirate Captain Kidd. But for 12 years, everyone has told him the stories are rumors. Now George has discovered the truth. And the truth is hidden in the web of tunnels mysteriously dug underneath his house in Manhattan. There are secrets to be revealed, friendships to be tested, and ancient injustices to be set right.
George and his friends will sink deep underground to a secret city in search of treasure, but they'll find they aren't the only ones looking for it - and there's much more at stake.