High in the Central Balkan Mountains of Bulgaria, Buzludzha Peak is the site of many significant events in Bulgarian history, including the first congress of the Bulgarian Social Democratic Workers Party in 1891.
In 1974, the Buzludzha Monument was constructed here, in concrete and shining glass mosaics, to commemorate that moment of history and the people who made it possible.
Even today its architecture feels futuristic, like a space ship from another world. Though it's officially closed, the monument remains a favourite of urban explorers in Eastern Europe.
The former German colony of Kolmanskop became the centre of the Namibian diamond rush in 1908, when a worker named Zacharias Lewala found a small rough diamond.
Over the next few years, the town and its wealth grew. There was soon a hospital, ballroom, power station, school, skittle-alley, theatre and sports hall, casino, ice factory and the first tram system in Africa.
When the diamonds ran out, and a new patch was discovered some 270km south, many of the town's inhabitants simply packed up and left. In the years since, the desert sands have swamped the town.
Gereja Ayam, Indonesia
Nicknamed the 'Chicken Church', this iconic prayer hall in the Javanese jungle was left unfinished, abandoned for years, until it found Internet fame.
Daniel Alamsjah devoted his life to building the multi-faith prayer hall after a religious vision. Designed to resemble a holy dove, many locals thought it looked more like a chicken.
Alamsjah ran out of money and faced ridicule for his design, until the strange structure was picked up by international media in 2015. The tourists his unique building attracted allowed him to carry on construction.
The Great Train Graveyard, Bolivia
The dry, dusty town of Uyuni, Bolivia, has long been one of South America's most important transport hubs. But today it's best known as the final resting place of hundreds of steam engines.
Many of these were imported from Britain during the 19th Century, when there were plans to expand the continent's transport network.
But the collapse of the mining industry left many bankrupt and those plans in disarray. Instead of paying to move the the trains, they were simply left to be eaten up the harsh winds of the salt flats.
Łapalice Castle, Poland
All is not as it seems at this ominous castle near Łapalice, Poland.These rotting ruins are actually still under construction.
Artist Piotr Kazimierczak only started building this structure in 1971. It was planned to be his personal studio, complete with swimming pools, ballroom, and twelve towers.
However his lofty ambitions met with some major problems: he ran out of money shortly after building the walls, and it turned out he'd built part of the castle on land he didn't own. Construction stopped, and local curiosity grew.
Haludovo Palace Hotel, Croatia
Built in 1971 and designed by architect Boris Magaš, this luxurious hotel on the Croatian island of Krk last welcomed guests in 2001. Today, while the building remains intact, the sci-fi interiors are destroyed.
The high, geometric ceilings, and white curving concrete mark this building as a modernist marvel. The hotel's casino, cocktail bar, swimming pools and saunas defied western expectations of eastern European hospitality.
Unfortunately, as the Yugoslav Wars broke out in the 1990s, the tourism industry collapsed. Throughout the conflicts, the hotel became home to refugees. The business never recovered, and closed in 2001.
Stromness, South Georgia
This small British island in the southern Atlantic Ocean, not far from Antarctica, is quiet today, but was once the site of major sealing and whaling stations. Thankfully, the ships are now gone, and the wildlife is safe again.
The Belltower, Lake Reschen
At first sight, this lone 14th-century bell tower, found near the village of Graun in northern Italy, doesn't appear like much...
...but it's the sole surviving building of the old village which, along with the nearby village of Reschen, was deliberately flooded in 1950 during the building of the artificial Lake Reschen.
When water levels are low or when ice covers the lake in winter, you can walk all the way up to the tower itself. Local legend says that on quiet nights you can still hear the ghostly ringing of bells.
The hill town of Craco in southern Italy, dates back nearly 1000 years, but was abandoned at the end of the twentieth century after a series of devastating landslides.
The evacuation of the town began in 1963 after a number of landslides, with many of the inhabitants moving down into the valley. In 1972 a flood damaged even more buildings, and in 1980 an earthquake destroyed what little was left.
The striking silhouette of the hill town and its eerie ruins have since become a sight for tourists and film crews.
This artificial hill overlooking the city of Berlin was made shortly after the Second World War, when an unfinished technical college was covered with demolition rubble. The US military then built a radar station on top of the hill, to spy on Soviet communications.
The listening station, officially known as Field Station Berlin, was constructed in October 1963, and operated until the fall of the Berlin Wall and the end of the Cold War. The equipment was removed, but the structure and protective domes remained, waiting to be explored.
Today, the building is an ad hoc tourist site, and covered in graffiti from years of urban exploration. You can take a guided tour, or wander alone, and the views across the city are well worth the climb.