Ski enthusiast Rainer Geissmann owns one of Switzerland's most unusual hotels. Buried deep within the Alps, Hotel La Claustra is fashioned from a former military bunker.

Though it boasts no view of Switzerland's pristine lakes or snow-capped mountains, last year about 500 guests paid 380 francs (S$540) to feast on a six-course dinner and experience doomsday living, James Bond-style.

"It is for those who want to try something adventurous - a hotel they have never seen before," said Mr Geissmann, 62.

The 17-room La Claustra is part of a quiet trend in Switzerland - adapting decommissioned military fortresses and bunkers for modern use. A few of the bunkers, originally intended as defence strongholds, were sold off by the Swiss military after the Cold War as high maintenance costs kicked in and the threat of armed conflict receded.

More than 50 now have been turned into museums, cheese factories, mushroom farms, and art or data storage centres.

Though no official figures are available, most of the country's 8,000 bunkers and military shelters have been bought, sealed off or set aside for historical preservation, Reuters has reported. Some masquerade as barns or mediaeval houses with painted-on windows, blending into the idyllic Swiss landscape.

La Claustra is just as unassuming. Located along the St Gotthard Pass connecting northern and southern Switzerland, the only thing that screams "stop" to cyclists or drivers is its red door. But beneath its ground floor is a 5,000 sq m hotel space complete with switchboard rooms, old military generators and ammunition storage shelves.

A cavernous interior frames the hotel's dining room, while ancient stairs lead to a weapon-loading deck. Guests open metallic bunker doors into cosy bedrooms fitted with army-green sheets. Pottery dots the facility, giving the cave an Asian monastic vibe.

"The place was unbelievable for me," said Mr Hausi Schlatter, 73, who visited La Claustra last summer. "The bunkers in my military days had no luxury, just pure stone, no heating. To see a bunker transformed this way is really something."

La Claustra was the brave experiment of Swiss architect Jean Odermatt, who designed and renovated the space for about 8 million francs in 1998.

But with a full service crew draining resources, it went bankrupt, and about five years ago Mr Geissmann picked it up for 1 million francs and went staff-light.

"I wanted a product so unique it has no competition. This hotel is one of a kind so I can control the quality and call the price," said the Liechtensteiner, who also owns a camper van park in Spain.

Now, he runs La Claustra with a cook, hiring more full-time staff only in the high season or when bigger groups are booked in.

Last year, about 4,000 people dined in the hotel, including a handful of Japanese, Korean and American tourists. But La Claustra's main clients are mostly vintage car club roadtrippers, bicycle or ski groups passing by or companies seeking an interesting seminar space.

Yet, this piece of real estate has its own quirks. "It's wet and humid - we know this problem and we fix it," said Mr Geissmann. He invests 2,000 francs a month to keep dehumidifiers and other equipment running, even without guests.

Due to its remoteness, the hotel may not be a huge profit churner. "But I earn enough to pay my bills," said Mr Geissmann.

Meanwhile, for companies like Swiss Data Safe, which houses data centres and stores valuables for clients, the physical security of military-grade bunkers is a big plus. "If bombs were to go off outside, assets in the bunker would not be affected," said Mr Dolf Wipfli, the firm's chief executive officer.

Built in the 1930s and 1940s, the bunkers and fortresses were equipped with cannon and machine guns to repel any World War II German invaders.

"At that time, Switzerland didn't have mobile armed weapons, no panthers (a type of mobile military tank), we simply didn't have the industrial infrastructure to build them," said Professor Rudolf Jaun, a retired history academic from the University of Zurich.

"So the Swiss practised area defence. The army would fight the enemy from these fortresses and bunkers fortified with weapons."

Most were strategically located along the mountainous regions of St Gotthard, St Moritz and Sargans, which were potential entry points or essential transport routes, Prof Jaun said. The bunkers were preserved and used in the Cold War, when the threat came from the Soviets.

At least 40 bunkers and forts now have been converted into museums. "Bunkers and fortresses are old school, a bit of nostalgia for the older generation; the young are less interested," said Prof Jaun.

For mushroom farmer Alex Lussi, 32, a dozen small bunkers turned out to be a golden opportunity to expand his business. Thanks to their insulating properties, bunkers are cool in summer and warm enough in winter so he saves money achieving the ideal mushroom-growing temperature.

"Who knew ex-military bunkers would one day turn out to be great places for farming fresh Swiss mushrooms," said Mr Lussi.