If you were to visit Wolin Island in the Baltic Sea, you could expect to see a coastal landscape, plenty of greenery and sandy shores.But underneath the welcoming, idyllic exterior lie the traces of an ominous chapter of Poland's past.Wolin Underground City, previously known as Vineta Battery, is a Second World War bunker built between 1935 and 1939.Made up of seven bunkers, the fortress was used by the Nazis to guard the coastline.Nowadays, the entire site is a museum that is open to the public and is constantly being expanded and restored.Wolin Island, which houses the underground site, is located in Poland's West Pomeranian region, which belonged to Germany before the Second World War - and was used by Hitler to ramp up defence.While the German army attempted to destroy the site at the end of the war, enough of it remains for tourists to take an inside look at history. Wolin Underground City is the remains of what was once a Nazi fortress With a kilometre of tunnels, the subterranean city is equal parts creepy and fascinating Wolin Island itself, by contrast, is complete with beaches, a national park and a lively ambienceIt remained a secret, known only by the highest-ranking officers of the Polish People's Republic, for years.However, the site opened to visitors in 2014 – and you need to go with a guide if you want to take a look around. There is a network of underground corridors almost a kilometre long, several rooms and shelters hidden under coastal dunes.Visits are interactive and truly bring history to life. You'll start by being organised in a platoon and observed by a 'commander'.Military trucks, fences, Soviet-style constructions and an eerie ambience welcome you in. The commander will then take you through the many rooms, tunnel and engine room shelter.Be warned, though – reports indicate the temperature never exceeds 10C (50F), so the air is chilly. That's not to mention the cold, lifeless interiors. Fences, signs and military vehicles welcome you in to the siteInside, there are also decoding stations, an underground kitchen, a medical room and a War Room command suite, according to CNN Travel. Nowadays, Wolin Island itself is a popular Polish holiday resort. It's home to sandy beaches, Wolin National Park and a rich Viking history.Seaside resort Międzyzdroje is popular among families, as is the nearby spa town of Świnoujście, which is near to the military bunker. Wolin Underground City is not the only military bunker you can visit nowadays – and you can even stay inside one of them.Nestled away in the rural village of Ringstead, in Dorset, is the Standby Generator Bunker – a one-of-a-kind Grade-II listed property.Dating back to 1941, the accommodation sleeps up to six across three bedrooms, all decorated in a steely, bare-brick wartime style with the original concrete walls left exposed in places.Located in the middle of the owner's dairy farm and only one field away from the sea, the bunker was part of RAF Ringstead's Chain Home radar, which formed part of Britain’s coastal early-warning radar system. It was used from 1941 to 1956.Similarly, last year it was revealed London's secret spy tunnels where Ian Fleming allegedly dreamed up James Bond are set to open to the public for the first time since the Second World War. Mannequins, as well as a real-life 'commander', demonstrate what the site would have been like during wartime You can get an insight into what the bunker would have looked like in actionThe London Tunnels, located in High Holborn, remained out of the public eye for almost 70 years under the Official Secrets Act.The attraction is set to house an intelligence museum, an interactive Second World War memorial and one of the world's deepest underground bars.Local authorities have approved plans to refurbish the 90,000 square-foot Kingsway Tunnel site. James Bond author Ian Fleming is believed to have worked in the complex as part of his role in naval intelligence during the conflict.