Underground places

Several hundred meters below the streets of the town Wieliczka snakes a real wonder of mankind: The Wieliczka Salt Mine, where miners since the 13th century have dug out more than 280 km of tunnels. Today the salt mine is UNESCO enlisted (along with Bochnia Salt Mine) and one of the biggest tourist sights in Poland. The tours will take you down 389 wooden steps (135 m), through damp tunnels all carved out of pure grey rock salt – and yes, you're welcome to check the taste for yourself by licking walls. There are artworks carved out of the rock salt by the miners through time, but the main attraction is the underground cathedral, also carved out of solid salt, with a highly polished salt floor, salt crystal chandeliers and a salt relief of the last supper. If salt is to your liking, you can even get married here.

During the Cold War of the 50s and 60s, the USSR feared an atomic attack from USA and built secret underground bunkers. Bunker 42 is such a bunker aimed for the military communication unit. Placed in a residential area underneath a dummy apartment block, it sits 60 meters underground and was staffed by more than 2500 people at its peak. After the Cold War and the fall of USSR, the bunker was no longer useful and got auctioned off. Today, it is turned into a Cold War museum where you get taken 18 floors underground and walked through the armored tubes and halls - and if you ask nicely, you might get to hold an AK-74.

This hike goes through six pitch-black tunnels with water channels high up in the jungle-covered mountains close to the Obo National Park. It's a 3-5 hours roundtrip back to the parking place (where the mark on the map is). The first bit takes you uphill through the jungle, while following a water pipe. The trail becomes narrow at some points with dangerous steep drops. Keep going to you get to a proper bridge with trails on both sides, do not cross but stick to the trail on the same side. A viaduct will soon cross the trail high above your head. Follow it to the first tunnel. Though the water runs in a separate channel, the path is muddy or just full of water. Angolares Waterfall will be right after the first tunnel. The second tunnel is really long with two bends and the third tunnel is full of small bats. Bring flashlight and proper footwear. It's a truly unique hike, which has no match anywhere in the world (to our knowledge).

Throughout the region, the Berber people are renowned for their resourcefulness in living in some of the most inhospitable places on Earth. They manage to find a harmony with the land to provide the essentials of life, like food and shelter. Nowhere is this more apparent then in the tiny village of Matmata. In order to stay cool in the sweltering mid-summer heat and warm in the surprising cold of winter, they have building their homes into the ground, literally. 5-10 meter deep artificial craters act as a central courtyard to rooms dug into the steep walls. The best way to experience this unique accommodation is to stay overnight. It's also the best way to experience the friendly village of Matmata. Most tourists show up on luxury buses, take pictures of the cave that was Luke Skywalker's home and leave. Spending the night seems to allow the village to open up to you, before getting closed into your cave room.

A landscape so imaginary and weird that it's hard to believe it's made by nature. The soft underlaying volcanic rock has been shaped by time, wind and water and left in the most fantastic formations with harder boulders balancing on top of tall rock chimneys. To add further magic to the place the mountain sides and underground have been dug out for housing, monasteries and even whole underground cities for thousands of years. It was in these multi levelled underground complexes where the Christians hide from the Romans to escape their prosecution. Some of the cities are more than 80 meters deep with eight floors and could accommodate more than 20.000 people. It's a truly unique place and you can even sleep in some of the many cave hotels in the area.

Underneath several gold-domed churches are two monastery caves, upper Lavra and lower Lavra. Each is an underground pathway with several niches where coffins and mummified monks are on display for public curiosity and worship. It is a strange religious tourist attraction where candle sellers also boast kitsch souvenirs like plastic icons. The passageways are pitch dark so you need to buy a candle to light your way, which can seem a bit dangerous when the tunnels get jam-packed with pilgrims. Though some visitors just come for the novelty factor, most consider it holy and a place for miracles. They cross themselves, kneel down for a short prayer and some might even kiss the glass cases containing a dressed corpse of a deceased monk. Photographing in the caves is not allowed, as you would have guessed from the above ground picture.

The foot tunnel under the Thames between Greenwich and the Isle of Dogs was first open in 1902. It was built so the workers living on the south side of the Thames could get to their workplace at the docks and shipyard on the north side. The 300 m long pedestrian tunnel is still functional today and is probably the most unusual way to cross the Thames.

These are the tunnels that the Viet Cong used to fight the South Vietnam alliance - including the Americans - under the Vietnam War (which the Vietnamese, by the way, call the American War). Two areas can be visited, Ben Duoc and Ben Dinh. All the tunnels that are open have been expanded to fit western-sized people, but you still need to crawl on your hands and knees to get through them. Over ground, the sights have turned into a tourist circus with souvenir shops and shooting range (yes, you can shoot with AK47s), but the old-school propaganda video and the tunnels give a pretty good insight to the horror of the war.