Believe it or not, a shocking number of expensive abandoned megaprojects, from entire metro systems to top-secret military bases, lie buried deep underground in numerous locations around the world. Click or scroll through for a trip into the bowels of the earth as we uncover 15 supersized subterranean structures that were left to rot.

Dubbed the Hollywood Subway, LA's first underground railway line opened in 1925 to much fanfare. Part of the privately owned Pacific Electric Railway, which at the time was the world's largest electric interurban railway, the near mile-long tunnel connected Beverly and Glendale Boulevards in Westlake with downtown, and was built at a cost of $4 million, the equivalent of $58 million (£44.2m) in today's money.

Passenger numbers peaked during the Second World War but plummeted as LA sprawled out and commuters increasingly turned to cars. The last train pulled out of the tunnel in 1955. While parts were filled in, other sections were used as a disaster shelter and film set, and became a magnet for the homeless, graffiti artists and urban explorers. These sections have since been sealed off and the main terminal building is now a luxury condo development.

This sizeable underground station and tunnels, which stretch for around half a mile, are located under Washington DC's Dupont Circle. Designed for the capital's streetcars, the station and tunnels cost $6 million and were built using the cut and cover method. Predating the DC Metro by several decades, they opened in 1949.

Like LA's Hollywood Subway, the Dupont Circle trolley station and tunnel fell victim to the rise of car culture and closed in the early 1960s when the streetcars were phased out. Parts of the tunnels were repurposed as a fallout shelter but sealed off in 1975. The tunnels were reopened to the public in 2016 and the station has since been transformed into an art gallery and exhibition space.

The project cost $7.9 million, around $69 million (£53m) in 2020 dollars, and consisted of 21 tunnels spanning 1.9 miles. It was abandoned in 1966 after Denmark protested and experts realised the ice cap was less stable than they original believed. Tons of toxic waste remain at the site, which worryingly, is moving ever closer to the edge of the ice cap.

Most citizens of Cincinnati have no idea an entire subway system exists beneath the city's streets. America's most extensive abandoned metro, the Cincinnati Subway was a colossal failure. Construction commenced in earnest in the early 1900s. Just over two miles of tunnels were built along with four stations before rapidly escalating costs forced the authorities to pull the plug in 1924.

The doomed megaproject is estimated to have set back the City of Cincinnati $13 million, which is the equivalent of $305 million (£232m) in today's money. Various proposals to reuse the tunnels and below-ground stations have been floated over the years but none have come to fruition, and the subway, which is now a haunt for the homeless and urban explorers, remains derelict.

The City of Chicago has been accused of literally burying piles of taxpayer's cash after it ploughed $400 million into the Block 37 CTA Superstation in the heart of downtown. Incredibly, the city pressed ahead and constructed the station despite having no guarantee that the service the enormous station was intended for, a futuristic airport link proposed by Elon Musk's The Boring Company, was feasible.

Work began on the superstation, which would have been the downtown terminal for express trains from O'Hare International Airport, back in 2005. It soon became apparent though that the express service wouldn't come to pass and the city halted building work in 2011. The superstation remains unfinished. In total the disastrous megaproject is slated to have cost taxpayers $400 million in 2015 dollars, the equivalent of $431 million (£328m) in 2020.

Costing $359 million, the top-secret structure was commissioned by the Chinese government when Cold War tensions were at their peak during the 1960s. The facility housed a nuclear plant capable of producing weapons-grade plutonium. When the Cold War warmed in the early 1980s, the project, which was only 85% completed, was cancelled and today the abandoned complex serves a thriving tourist attraction. The cost of the megaproject would cost $883 million (£673m) in today's money.

With the Second World War turning heavily in favour of the Allies, the Nazis set about building a gargantuan underground city in April 1943. Shrouded in secrecy, Project Riese ('riese' is the German word for giant) involved the construction of seven underground structures connected by 5.5 miles of tunnels under the Owl Mountains in modern-day Poland.

Thousands of prisoners of war including children were forced to work on the mysterious megaproject and many died in the process. Construction continued until May 1945 when construction, which is estimated to have cost $60 million at the time, abruptly ceased. To this day the exact purpose of Project Riese is unknown but some historians have speculated that the subterranean city could have been built as a bunker for Hitler, or to accommodate a military base.

There was just one very major catch. While the border with Luxembourg, Germany, Switzerland and Italy was robustly defended, the frontier with Belgium had little if any protection. The Nazis exploited this weakness and invaded France in May 1940 via the Franco-Belgian border. The Maginot Line was manned once again after the Second World War, but it was rendered obsolete during the Cold War and effectively abandoned in the 1960s.

Read about other abandoned megaprojects that turned out to be billion-dollar wastes of money

Carved into a mountain, this covert naval base near Tromsø was built by the Norwegian Navy in the late 1960s at a cost of $494 million, which is about $3.8 billion (£2.8bn) in today's money. No expense was spared on the base, which was equipped with the most advanced and sophisticated technology money could buy, and featured a plethora of tunnels and rooms beneath the mountain. It took 30 years to build.

The Bond villain-esque facility acted as the main service base for NATO submarines operating in the Norwegian and Barents Seas. But Olavsvern lost its key strategic importance following the fall of communism and was deactivated in 2002, before its eventual closure in 2008. In an unusual move the base was put up for sale on a Norwegian auction website and was sold at a knockdown price in 2011 to a consortium of Norwegian oil companies. Much to the Norwegian government and NATO's chagrin, the base has since been rented by affiliates of the Kremlin-linked Russian energy giant Gazprom.

Nicknamed the Deserton, the Superconducting Super Collider (SSC) in Texas would have been America's very own particle collider, rivalling the famous particle collider at the European Organization for Nuclear Research (CERN) in Switzerland. If it had been completely built, the SSC may well have beaten CERN to discover the Higgs boson 'God particle'. 

The SSC was first proposed in 1976 and construction began in the late 1980s. By the early 1990s, costs were spiralling out of control. The powers that be questioned the viability of the project and whether it was worth the monstrous expense. Needless to say, the shockingly over-budget SSC was cancelled in 1993. By this time, the cost of the megaproject had skyrocketed to $2.4 billion, the equivalent of $4.3 billion (£3.3bn) today, which is quite a cost for an empty underground tunnel in Texas.

A bewildering waste of taxpayers' money, the Stanley R. Mickelsen Safeguard Complex in Nekoma, North Dakota was completed in April 1975 at a cost of $6 billion (£4.6bn) when adjusted for 2020 dollars. The cutting-edge complex, which was was built beneath and around an eye-catching pyramid structure, provided launch and control for scores of short and longer-range anti-ballistic missiles. 

Discover the world's most expensive abandoned military bases

When it was confirmed that nuclear power stations could store waste on site safely over decades, politicians began to wonder if the project was even necessary. President Obama pulled federal funding in 2011 and effectively canned the project. By then around $15 billion had been invested, the equivalent of $17.1 billion (£13bn) in today's money. With no real plans afoot to revive the project, the Yucca Mountain repository remains in limbo.

The base, which is estimated to have cost an astronomical $6 billion at the time, the equivalent of $48.7 billion (£37.2bn) today, was operational until the early 1990s. Despite the vast expense, the withdrawing Yugoslav People's Army destroyed much of the complex during the Yugoslav Wars in 1991 to prevent the opposing forces from using it for their own ends. The complex has remained abandoned ever since. 

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