How train tunnels beneath Amsterdam revealed a medieval treasure trove

Building Amsterdam's North-South metro line was big trouble – a budget-blowing 15-year operation that involved carefully burrowing beneath the foundations of centuries-old architecture.

For archaeologists tasked with sifting through soft mud to preserve any history disturbed by the massive engineering project, it was no easy feat either. Their potentially hazardous work took place inside concrete boxes pressurized to keep out deluges from the Dutch capital's ubiquitous waterways.

Today, the fruits of their subterranean labors can be seen at Rokin station, one of eight stops on the route and one that doubles up as an impressive underground archaeological museum, with nearly 10,000 artifacts on display.

The station, well worth visiting in its own right, is a testament not only to the rich heritage on which Amsterdam is built, but also to the engineers and archeologists who worked so hard to preserve it.

The fruits of their labor are displayed in two glass cases positioned between the escalators, one case at either end of the station. On any given day, it's not unusual to find a commuter going up and down the escalators, just to get a better look.

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A significant number of these artifacts were found in and around Rokin, a neighborhood that lies along the city's main Amstel river that was at the heart of Amsterdam as it developed from the 13th century onwards.

Waterways tend to become dumping grounds, accumulating objects over the centuries. The Amstel riverbed around Rokin was no different.

"The sheer mass of material we unearthed during the construction of the North-South line was extraordinary," says Peter Kranendonk, one of two senior archaeologists leading the excavations during the metro project.

"The construction gave us a unique opportunity to excavate under the city up to a depth of 30 meters," he adds. The oldest items found were mollusc shells dating to over 115,000 years ago.

The artifact displays at Rokin station are organized into various themes. In the north display, the focus is on objects related to food, science and technology, arms and armor, communication, games and recreation, personal artifacts and clothing; while the south display includes items from buildings and structures, interiors and accessories, transport, as well as craft and industry. All of these artifacts provide insights into Amsterdam's glorious, and sometimes unknown past.

"Some objects, like the 500-year-old coins, have a direct story behind them," says Kranendonk. "On the basis of the finds, we can also say something about the use of an area," he adds.

In one spot at Rokin, unearthing a concentration of chopped animal bones pointed to the existence of a butchery nearby in the 17th and 18th century. At another spot, an abundance of furniture fittings confirmed the presence of a furniture maker's shop in the 19th century.

"Prior to the excavation of these artifacts, the city had an archaeological archive of only about 70,000 artifacts," says Hoite Detmar, who served as the director of the North-South metro project from 2016 till its completion. "We found 10 times as many during the construction of the North-South line."

Kranendonk elaborates on the rather unconventional excavation process behind these finds.

"This was not a normal dig," he says. "Usually, excavation is done before building. But in this case, the construction plans were already finalized. So we had to become part of the existing process. The civil engineering team were building and we were excavating."

For the archaeology team, working in the caissons was a novel experience. A caisson is a large watertight concrete chamber, open at the bottom, from which water is kept out by air pressure and in which construction work is carried out underground or underwater.

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"It was an interesting experience but also a bit frightening," says Kranendonk. "The deeper you go, the more compressed the air gets. It's like deep sea diving," To acclimatize to the caissons, teams had to spend time in a pressure chamber before entering and after exiting, otherwise they’d face risk of "the bends," when gas bubbles form in the body, potentially leading to paralysis.

To enable people to engage with the Rokin displays at leisure, an online database of nearly 20,000 objects, Below the Surface, was created, providing information about every single item in the glass cases. "It's a process of discovery in its own way," says Kranendonk.

A documentary about the excavation called "Amstel, Spiegel van de Stad" (Amstel, Mirror of the City) and a beautiful coffee-table book, "Amsterdam Stuff" were also created.

"We knew we would be working in the city for a very long time and would inconvenience citizens a lot," says Hoite Detmar. "This was one of the many ways we gave back to the city."

In addition to the two archaeological displays, the walls of Rokin station adjacent to the tracks are covered with stone mosaics by artists Daniel Dewar and Grégory Gicquel depicting 33 of the artifacts unearthed – a keyboard, a pike, a teapot, dice, a butterfly, among others.

There is even a mosaic of a crocodile which represents a crocodile jaw that was unearthed, a rather unusual find for this part of the world.

Hailed as one of the most challenging infrastructure projects in the Netherlands, the North-South line was inaugurated in July 2018, with a lot of fanfare. The route is six miles long – of which 4.5 miles are underground – and runs under the historic city center, the Central train station, and the IJ, a water channel which separates the city's north from its center.

The line linked neighborhoods like the northern suburbs (previously unconnected by rail) to the city center, eliminating the need to take a ferry across the IJ or to drive through the IJ tunnel. It also halved the 30-minute travel time required to traverse the city from north to south. Right after the opening of the line, an estimated 120,000 commuters used it every day.

However, the initial plans for the North-South line were not met with enthusiasm. Public resistance to this project was prompted by the traumatic experiences during construction of Amsterdam's first metro, the East Line, in the 1970s. A large part of the Nieuwmarktbuurt neighborhood was demolished to make way for the project, leading to anger and riots in 1975.

The construction of the North-South line began in 2003, one of its key objectives being to preserve the existing built environment.

With this in mind, a specific route was chosen and several new construction techniques were used, including the deployment of a customized tunnel boring machine, which made it possible to dig deep in Amsterdam's soft soil, without impacting structures above.

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However, public concerns about homes collapsing loomed over the project. In June 2008, work came to a grinding halt when four 17th-century buildings near Vijzelgracht station sank by about 25 centimeters (10 inches), rendering them uninhabitable.

"Thankfully, no one was injured," says Detmar. An independent assessment was conducted and work resumed in the summer of 2009. The heritage houses were also restored.

The project was beset with many engineering challenges that led to the doubling of the construction budget from 1.4 to 3.1 billion euros. The initial launch date of 2011 was also pushed back to 2018.

Despite these challenges, the North-South line has functioned smoothly since launch.

Detmar says he's pleased with the appreciation the project has received to date, especially for the art at each of the eight stations on the new line.

Rokin is the highlight.

"When I travel to Rokin station, I see people really studying the archaeological exhibits," he says. "I hope more people will take the metro to see this underground museum."