Cage of Eden, Vol. 20

After learning that the island they're on may be near Okinawa, Sengoku and his friends put a plan in motion to sail back to Japan. But before they can do that, a decision is made to check our the fourth and final tower on the island. Their journey into the depths of this mysterious structure is sure to result in many discoveries, but then they least expect it, they may also find that they're not the only ones in the building…