Norway's proximity to the USSR during the Cold War led to it building many military bunkers – some of them vast secret bases for planes and ships. Tensions with Russia have brought the bunkers back into focus.

Tourists in their hundreds of thousands visit northern Norway each year. But there is a secret world they never see. For hidden away in mountain caverns are jet fighters and nuclear submarines.

Norway is a land with many bunkers. At the peak of the Cold War, the sparsely populated, mountainous country had around 3,000 underground facilities where its armed forces and allies could hide and make life difficult for any invader. Dating back to when the Scandinavian country was part of Hitler's Atlantic Wall during World War Two and even earlier, their existence was barely known to the Norwegian public.

Now as a European war engulfs eastern Ukraine, Norway is reactivating two of their most iconic underground structures of the Cold War.

Close to Norway's border with Russia north of the Arctic Circle, the hangars of the Bardufoss Air Station and the naval base at Olavsvern feel like they belong in a spy film, with their rough rock walls, gleaming concrete and military equipment. Carved out of a mountain side, protected by around 900ft (275m) of tough gabbro rock, the Olasvern base is particularly evocative with its 3,000ft-long (909m) exit tunnel complete with massive blast door.

Why are these huge bunkers needed today? The Soviet Union – the reason they were built in the first place – no longer exists. Does it really make sense to pour money into such expensive structures?

In the publicity shots for the reactivation of Bardufoss hangars, the Lockheed Martin fighter – the F-35 Lightning II – perches menacingly like a bird of prey under the spot lights of the hangar's arched roof. Opened in 1938, the air station was once used by German fighters protecting the giant battleship Tirpitz while it was anchored in a nearby fjord.

After the war, the Royal Norwegian Air Force then used its mountain hangars to protect its fighter planes from a possible Soviet attack. These hangars included everything the planes and their pilots needed, such as fuel storage, weapon storage, space for maintaining the aircraft systems, and crew areas. Then around 40 years ago it was closed down and mothballed.

Now, Bardufoss looks like it may be needed once more.

The role of the reactivated base which has had structural and equipment upgrades is to help the "resilience and survivability" of Norway's F-35s in the face of a Russian attack. The Russian invasion of Ukraine has shown the world how vulnerable expensive military aircraft like these $80m–$110m (£64m to £80m) F-35s can be when on the ground, particularly to attacks by "kamikaze" drones that can cost as little as $300 (£230).

Instead of placing tyres on wings or constructing hangars out of

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