Pendem Cilacap Fort, Fortification and museum in Cilacap, Indonesia

Benteng Pendem Cilacap is a pentagon-shaped Dutch coastal battery of brick covered in plaster, its buildings sunk 1–3 m under earth so they resemble mounds. The site holds a barracks with fourteen chambers, a two-storey ammunition hold, a 113 m tunnel with four entrances, a clinic and a prison, plus a 500 m excavated moat and 11 cannon positions.

Construction of the coastal battery began in 1861 and it was operational by 1879, built by the Dutch East Indies government to defend the port of Cilacap. Japanese forces occupied it from 1942 to 1945, and Allied troops held it until 1949. Between 1952 and 1965 the Indonesian Army used it, with Kopassus training there from 1956 to 1962. It was excavated in 1986 and opened to visitors in 1987.

Locals call it benteng pendem, Javanese for "buried fort", because earth and sand covered the structures until excavation began in 1986. The site is said to be haunted, and rooms have been used for bravery tests in which a person is left alone without lights for an hour.

The fort is pentagon-shaped, not square, and its brick-and-plaster buildings sit under 1–3 metres of earth, so the excavated moat, 500 metres long and 2–3 metres deep, is the clearest way to orient yourself. The tourist entrance on the east side was originally the rear of the fort. Guides are available on site.

The fort's prison held both civilian and military detainees under Dutch rule. A 113-metre tunnel with four entrances links parts of the structure, and a two-storey ammunition hold survives; guides say further rooms, perhaps a tunnel to Nusa Kambangan, remain buried.