Switzerland once had thousands of bunkers hidden in the Alps to guard against foreign invasion during the Second World War. During the Cold War, they remained as bomb shelters in case of nuclear attack.

But with those threats long gone, many of the decommissioned fortresses now serve as everything from cheese and mushroom factories, to museums and hotels.

Underground hotel nestled into rock

It's not apparent from the outside, but this four-star hotel, converted from an underground artillery fort, has 17 rooms, a swimming pool, library, spa and restaurant in Switzerland's Gotthard Pass region.

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Hotel La Claustra was once a stronghold of the Swiss army. (Arnd Wiegmann/Reuters)

Subterranean, but not claustrophic

Designer Jean Odermatt included an eery view from the hotel's restaurant.

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Hotel La Claustra is carved into mountain rock 2,050 metres (6,726 feet) above sea level. (Arnd Wiegmann/Reuters)

Cheesy solution to former ammunition bunker

Swiss cheesemaker Seller Kaeserek AG uses this former bunker to produce Raclette cheese in the town of Giswil.

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The space was once used for storing ammunition. (Arnd Wiegmann/Reuters)

Elsewhere, it's mushrooms

In 11 former bunkers the, company Gotthard-Pilze produces some 24 tonnes of shiitake mushrooms per year. 

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Conditions are right for shiitake mushroom to grow inside a former ammunition bunker near the town of Erstfeld. (Arnd Wiegmann/Reuters)

Turret-mounted long guns

This one-time artillery fort was meant to defend Switzerland against a Nazi invasion and was built in 1942. 

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A 10.5-cm gun stands over the town of Faulensee. The fort was in military use from 1943 to 1993 and is now open to the public as a museum. (Arnd Wiegmann/Reuters)

Ready for an invasion that never happened

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The fortress in Faulensee housed four 10.5-cm guns. (Arnd Wiegmann/Reuters)

Artillery control room-turned-museum

This one-time bunker in Faulensee was operational from 1943 to 1993.

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With the threat of foreign invasion a thing of the past, thousands of military bunkers and fortresses in Switzerland have been put to commercial use or turned into museums. (Arnd Wiegmann/Reuters)

Swiss defence included tunnels

This tunnel connects underground bunkers in the town of Faulensee. The system was built to defend against an invasion by German forces that was planned but never carried out. 

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Switzerland once had a network of thousands of gun forts and bomb shelters. (Arnd Wiegmann/Reuters)

Sasso da Pigna artillery fort at 2,106 metres

Construction of this gun fortress took place from 1941 to 1945, and was in military use until 1999. Since 2012, it has been open to the public as the Sasso San Gottardo museum. 

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Here, the muzzle of a 15-cm gun is tucked into the rock at the St. Gotthard mountain pass. (Arnd Wiegmann/Reuters)

Nothing out of the ordinary here

But what appears to be a stable was actually once a machine-gun fortress called Fuchsegg. It stands beside the Furka mountain-pass road near the village of Realp in the central Swiss Alps. It was built in 1943 and remained in military use until 1993.

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On the left is a former artillery fortress camouflaged to look like a barn. (Arnd Wiegmann/Reuters)

Camouflage, Swiss-style

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A former infantry bunker is disguised as a medieval house in Duggingen. (Arnd Wiegmann/Reuters)