Rushima

Rushima is an island located in Tokyo Bay, and a suspected headquarters of Aogiri Tree.

History
During the Second World War, the island was fortified and used as a base to defend the capitol from attack. However, it was abandoned after the war and became uninhabited. The island was closed to the public, but apparently still used by those willing to sneak out there to enjoy the ocean.

Aogiri Investigation
Rushima comes under suspicion after witnesses report seeing people gathering on the island, leading the CCG to investigate it. The Hachikawa Squad is sent to scout the island, with the mission of verifying the presence of Aogiri.

Trivia

 * It appears to be based on Sarushima, a real island located in Tokyo Bay.