A number of underground fortresses along the banks of the Linth and Seez canals in northern Switzerland have begun opening their armoured doors to the public.

The military installations were carved out of the rock deep underground and date back to the Second World War. They were specially constructed to defend Switzerland in case of a Nazi invasion.

The Grynau fortress in canton Schwyz was the first to open closely followed by the Sargans fortress in canton St Gallen. Tours are offered through their underground labyrinths.

A visit to either place reveals artillery cannon, and a vast network of underground caverns and rooms which housed ammunition stores, barracks for the infantry and a secure place for military planning. Up until five years ago, civilians were still unaware of their existence.

Nowadays, amidst more open security in Europe and cuts to the Swiss army, the fortresses were seen as surplus to requirements. Aware of their historical value, private foundations have stepped in and begun restoration programmes.

It's estimated that there are around 400 such installations in the area. There are eight other planned fortress openings for this year to allow visitors a glimpse at the intricate arrangements of cave dwelling army style.

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