Sarushima (Monkey Island) — Tokyo Bay Fortress Battery Ruins

Sarushima (猿島, "Monkey Island") is a small uninhabited island located approximately 1.7 kilometers offshore from Yokosuka New Port in Kanagawa Prefecture. It is the largest natural island in Tokyo Bay and holds significant military historical value as a coastal defense fortification spanning from the late Edo period through World War II.

The island's military history began in 1847, when the Tokugawa shogunate constructed a battery on the island to defend Tokyo Bay against the growing threat of foreign warships — making Sarushima Japan's first daiba (battery island). In 1853, Commodore Matthew C. Perry surveyed the island during his expedition to open Japan, naming it "Perry Island," though the name never gained acceptance. The battery suffered significant damage during the Ansei Earthquake of 1855.

Following the Meiji Restoration, the island's strategic importance grew. In 1877, Sarushima came under the jurisdiction of the Imperial Japanese Navy, and in 1881 it was transferred to the Army, which initiated a comprehensive fortress construction program. Completed in 1884, the fortress featured brick-lined tunnels, ammunition magazines, gun battery emplacements, barracks, and powder stores — all constructed using the distinctive Flemish (French-style) bond bricklaying technique with high-quality red bricks produced in Aichi Prefecture. The fortress was ingeniously designed by excavating downward from the island's summit to create a network of tunnels and magazines invisible from passing ships.

The island sustained damage again during the Great Kanto Earthquake of 1923, after which the Navy undertook reconstruction work in 1925. As part of the Tokyo Bay Fortress system (東京湾要塞), which comprised 32 batteries and sea forts across the Miura and Boso peninsulas, Sarushima played a critical role in defending both Yokosuka Naval Base and the approaches to Tokyo.

During World War II, anti-aircraft guns were installed on the island in 1941 to defend against Allied air raids. Throughout the war, Sarushima remained an active military installation. Following Japan's surrender in 1945, American occupation forces took control of the island and removed all functional military equipment, ending Sarushima's nearly century-long history as a fortress.

The island was returned to Japanese control in 1961 and was transferred from national to Yokosuka city administration in 1995. Today, five battery emplacements remain visible, along with extensive brick tunnels — including a 90-meter passage now popularly known as the "Tunnel of Love" for its atmospheric, dimly lit interior. The red brickwork represents one of only four surviving examples of Flemish-bond military construction in Japan. In 2015, Sarushima was designated a National Historic Site as part of the Tokyo Bay Fortress Ruins (東京湾要塞跡 猿島砲台跡).

The island is accessible year-round via a short 10-minute ferry ride from Mikasa Terminal in Yokosuka. Its overgrown brick passages and gun emplacements, draped in moss and vines, have earned comparisons to the floating castle in Hayao Miyazaki's "Laputa: Castle in the Sky."