Vast Underground Spaces and a Hidden History Lie Beneath Maastricht

Travelers know Maastricht, tucked along the southern border of the Netherlands, for its cobbled streets, stately 17th-century townhouses and the remnants of its fortifications, including bastions, towers and medieval gates. But some of the city’s most eye-catching locations are underground.

These subterranean sites are among the roughly 500 quarries scattered throughout Limburg — the hilly region of the Netherlands that’s home to Maastricht — and Wallonia, in neighboring Belgium. There is evidence of mining activity here dating to Roman times and the underground limestone quarries have been in use since around 1300.

Long after limestone ceased to be cost-effective as a building material — the Maastricht quarries were fully abandoned in the 1920s — these sites have taken on new roles. During World War II, some served as air raid shelters for the local population and artistic treasures from Dutch museums.

“The only thing all these structures have in common is that they’re underground,” explained Jos Notermans, a member of the Menno van Coehoorn Foundation, which helps preserve the Netherlands’ old fortification systems. “They each have their own story and characteristics,” he added.

Notermans has long been drawn to the world beneath Maastricht and its environs. “Like a lot of youth in Maastricht I was attracted by the dark underground corridors. I didn’t do any mischief, but I started studying,” he said in a phone interview.

He isn’t the only one fascinated by the area’s subterranean features. According to the SOK, a study group affiliated with the Royal Society of Natural History in Limburg, there is evidence of guided excursions through the region’s quarries dating back to the 17th century, at least.

Nowadays, most of the caves are off limits, with trespassers subject to hefty fines. However, an official company, Maastricht Underground, offers several tours of these otherwise inaccessible locations, where labyrinthine passages, illuminated primarily by flashlights and hand-held lamps, reveal the city’s hidden history.

Underground Art

The imposing Fort Sint Pieter was built between 1701 and 1702 to shore up the city’s defenses in the wake of the Siege of Maastricht by Louis XIV of France. Recently restored, the fort sits on the St. Pietersberg hill. Beneath it lies a network of tunnels that was the world’s largest subterranean complex until the 20th century, according to the SOK.

When the fort was constructed, its gallery walls were whitewashed to reflect the light of lanterns, and they still glow brightly as visitors pass through with modern lamps. The brick fort has an ingenious and efficient system of embrasures, slits in the walls for musketeers to shoot at potential invaders, and its cannons were capable of hitting specific targets up to 1,600 feet away and 10 times that distance to intimidate an enemy that tried to take the hill.

Of the 20,000 tunnels that once made up the limestone quarry beneath the St. Pietersberg hill, roughly half are intact today. The North Caves, beneath Fort Sint Pieter, are filled with charcoal drawings and doodles left by the caves’ various workers and visitors throughout the centuries. One of the largest depicts the mosasaur, a prehistoric reptile that went extinct around 66 million years ago, and whose skull was discovered in these caves in the late 18th century.

During World War II roughly 800 artworks were deposited here, protected from bombs by a hundred feet of mountain in a custom-built storage facility that was known as the Kluis, or vault, a concrete bunker with a climate control system. Arguably the most famous painting kept here was Rembrandt’s “The Night Watch,” which was detached from its canvas, rolled up and stored in the Kluis for three years.

The Zonneberg Caves, part of the same quarry system underneath St. Pietersberg hill, are laid out more regularly that the tortuous North Caves, with some tunnels more than 600 feet long. “It’s more like a New York street grid,” explained Ed Houben, 55, who has given underground tours in Maastricht since 1995.

The complex also contains the oldest charcoal markings found in Maastricht’s quarries. On his tours, Houben points out two pictograms that date from 1551. And during World War II, the Zonneberg Caves were readied for a potential mass evacuation, with a well, a functioning medical center, a bakery, a chapel and enough room for the city’s entire population of roughly 50,000.

Defensive End

At the west end of the city center lies a network of 247 tunnels known as the Waldeck Casemates. This labyrinthine defensive system, built in the late 16th century and enlarged and fortified over the next 200 years, was designed to deter and repel invaders.

Soldiers would sit and observe plates of water or dried peas on a drum skin; if there were vibrations on the water’s surface, or the peas started shifting, they could tell the enemy was approaching by burrowing underground. Miners stationed in the casemates then dug new tunnels of their own and filled them with gunpowder to ambush the intruders.

During World War II, many of these tunnels were turned into air raid shelters. They were outfitted with wooden benches and lit with bicycle lights, with space for up to 25,000 people.

The strangest and most eerie of Maastricht’s underground sites is the former NATO control center located under the Cannerberg, a hill just outside town. “We call it a mountain, but it’s obviously not the Himalayas,” said Houben of the roughly 350-foot hill on the Dutch-Belgian border. (Outside his work as a tour guide, Houben has also gained some attention separately as an international sperm donor. “In a past life, I had some unusual extracurricular activities,” he said in a phone interview.)

The NATO Headquarters Cannerberg / Joint Operations Center was a sprawling complex used during the Cold War. From 1954 to 1992, NATO coordinated West German air defense and military operations — the German border lies less than 20 miles from here —from this underground control center whose five miles of corridors once housed offices, control rooms, two restaurants, a barbershop and a small golf course with artificial grass. “They had everything you need,” said Notermans.

Though officially classified as secret, the headquarters’ existence was well-known, especially after activities ramped up here in the wake of the Cuban Missile Crisis in 1962.

“It was probably the most intensely used cave in the whole area because until ’92 you had 250 to 300 people working there on a daily basis, 365 days a year,” said Notermans.

“People knew soldiers were going there. Occasionally, a helicopter landed there. The secret wasn’t that there was a military operation, but what they were doing inside,” he added.

Rumors circulated about nuclear warheads being stored there. Though untrue, those rumors might have served as deterrence. The NATO caves were also intended to be a shelter in case of a Soviet attack.

“They made preparations to keep people safe against N.B.C. — nuclear, biological and chemical weapons — and all those provisions have been dismantled. You can still see the air locks, but they don’t function anymore,” Notermans said.

Most of the infrastructure was stripped in the early 1990s, but as you tour the ghostly corridors named for the first seven letters of the NATO alphabet — Alpha Street to Golf Street — you can still view, by the glare of your flashlight, the ruins of the impressive complex, which include the original generator rooms and other areas that have been partially reconstructed, including control centers, an officer’s club called the Flintstones bar and the all-ranks watering hole, the Mushroom Club.

Notermans said that he had noticed a different type of interest in the caves since Russia’s full-scale invasion of Ukraine in 2022. “People are starting to ask: Could this all still be used if necessary in our times?” he said.

“We are lucky in Maastricht that we have underground structures,” he said. “You can compare that to the Metro in Kyiv, where people go to be safe against the bombs, but at the same time we don’t have any more protection against N.B.C. weapons,” he added.

There are tours of all five sites during this year’s TEFAF Maastricht. Visit www.exploremaastricht.nl/en for dates and to book tickets.