Kashima Naval Air Group Ruins (Oyama Lakeside Park)

The Kashima Naval Air Group (鹿島海軍航空隊) was established on December 15, 1938, on the southern shore of Lake Kasumigaura in Anjū Village, Oyama (present-day Miho Village, Inashiki District, Ibaraki Prefecture). It was designated a permanent practice air group (練習航空隊) under the Yokosuka Naval District, tasked with training seaplane pilots for the Imperial Japanese Navy.

The origins of the base trace to 1937, when the Navy began land acquisition and construction under the provisional name "Anjū Air Group" (仮称安中航空隊), filling in rice paddies and building military roads and water supply infrastructure along the lakeside. The site was selected because the Kasumigaura Naval Air Group's seaplane training section needed to relocate — its facilities at Tsuchiura were being repurposed for the newly established Naval Aviator Preparatory Course (予科練) training division, which opened on March 1, 1939.

Upon activation in December 1938, the Kashima Air Group began training pilots on floatplane types including the Nakajima E8N (Type 95 Reconnaissance Seaplane / "Dave"), Mitsubishi F1M (Type 0 Observation Seaplane / "Pete"), and Aichi E13A (Type 0 Reconnaissance Seaplane / "Jake"). A catapult training platform was installed to simulate shipboard catapult launches for pilots destined for cruiser and battleship seaplane detachments. Lakeside concrete slipways allowed seaplanes to be winched in and out of the water.

By December 1941, when the Pacific War erupted, the base had swelled to over 1,000 personnel with more than 100 aircraft. The air group's training mission expanded beyond basic flight instruction to include water-based reconnaissance techniques, anti-submarine patrol procedures, and formation flying.

As the war progressed and Japan's strategic position deteriorated, the Kashima Air Group was drawn into homeland defense operations. Pilots conducted anti-submarine patrols over the Kashima Sea (鹿島灘) off the Pacific coast of Ibaraki, and the base served as a staging point for reconnaissance missions.

In the war's final months, the Kashima Air Group organized a special attack (tokko) squadron — the 2nd Sakigake-tai (第二魁隊). On May 11, 1945, five Kashima aviators departed Ibusuki Air Base in Kagoshima Prefecture in two seaplanes: three men in a Type 0 Reconnaissance Seaplane (Aichi E13A) and two in a Type 94 Reconnaissance Seaplane (Kawanishi E7K). Their mission was to attack Allied warships off Okinawa. None returned. Additional Kashima personnel were transferred to Kyushu for special attack duties but were awaiting orders when the war ended on August 15, 1945. In total, several dozen Kashima Air Group members perished in combat, including those killed during Allied strafing attacks on the base itself.

After Japan's surrender, the base was requisitioned by the occupation authorities. In 1946, Tokyo Medical and Dental University established the Kasumigaura Branch Hospital on the site, repurposing the wartime buildings for medical use. The hospital operated until 1997, after which the site was largely abandoned and the structures fell into atmospheric ruin — earning it fame in Japan's "haikyo" (ruins exploration) community as one of the most photogenic abandoned military sites in the country.

In 2022, Miho Village selected a designated management organization ("Project Ibaraki") to preserve and manage the site. After crowdfunding and renovation efforts, the site reopened to the public on July 22, 2023, as Oyama Lakeside Park (大山湖畔公園). It is a war-ruin park where original naval base structures from WWII can be explored in a near-original state.

Surviving structures include:

• Headquarters Building (本部庁舎): A reinforced-concrete three-story structure with 30 cm-thick walls and 4-meter ceilings. Features symmetrical Art Deco-influenced staircases. Used as the hospital's main building from 1946 to 1997.
• Boiler House (汽缶場): Contains two original coal-fired heating boilers manufactured by Tokyo Tsukishima Ando Ironworks in 1939 (Showa 14) — said to be the oldest surviving coal-fired heating boilers in Japan. The brick smokestack rises 32 meters with a 3.2-meter base diameter.
• Self-Powered Generator Building (自力発電所): The base's backup power facility, retaining its original industrial infrastructure.
• Vehicle Garage (自動車車庫): Now serves as the park reception and visitor orientation area.
• Catapult Foundation (カタパルト跡): Concrete base of the training catapult used to simulate shipboard seaplane launches.
• Seaplane Slipways (滑走台): Lakeside concrete ramps for launching and recovering seaplanes, recognized as Civil Engineering Heritage (土木遺産) by the Japan Society of Civil Engineers.
• Fuel Storage Ruins (軽油庫跡): Remains of the base's diesel and aviation fuel depot.

The site has also gained recognition as a filming location for movies, television dramas, and music videos, its evocative ruins providing an atmospheric backdrop that few other Japanese locations can match.

A memorial stone erected in 1999 by surviving Kashima Air Group veterans and volunteers commemorates the fallen, emphasizing the importance of preserving wartime history for future generations. The inscription honors those who died with "burning devotion to the salvation of this country."