What Are the Biggest Underground Structures?

Underground structures are challenging to build. When we build down rather than building up, issues like soil science, structural stability, and waterproofing all become increasingly important.

As experts in basements, crawl spaces, and foundations, we’re familiar with the unique challenges of underground construction. Yet around the world, there are amazing underground sites that are engineering marvels. 

In the list below, you can read about some of the biggest, deepest, and most impressive underground structures. 

Deepest Subway Station

Arsenalna in Kyiv, Ukraine: 346 feet below ground

You may be used to heading underground to catch the subway, but in Kyiv, Ukraine, it takes a five-minute escalator ride to reach the Arsenalna metro station. 

At 346 feet below ground, it was the deepest station in the world when it opened in 1960, and it still holds the title today. 

The depth is a result of the hilly landscape. The town of Arsenalna is at the top of a hill next to a steep valley and the Dnieper River. The Arsenalna station is far below the surface, but the next station on the line is at ground level. This allows the underground train to travel on a mostly level path, even though there’s a large hill above ground. 

Comparatively, the deepest subway station in the NYC subway system is the 191st Street Station in Washington Heights. It’s 173 feet underground, about half as deep as the Kyiv Station.

Deepest Basement

Sydney Opera House, Australia: 120 feet underground

The curved roofline of the Sydney Opera House makes it one of the most distinctive buildings in the world. However, below street level, there’s a massive underground parking garage that’s been named as the world’s deepest basement. 

The 12-story parking garage extends 120 feet below the ground. For an additional challenge, the parking garage is only about 200 feet from the east side of Sydney Harbor, and it extends about 91 feet below sea level. 

Interestingly, the concrete structure is not a typical underground square. Even though early designs called for a standard parking garage, regulations around fire escapes were creating a design challenge that made it difficult to maximize parking spaces while also ensuring safety. 

The final solution was to create a structure that was a series of underground curves like a double helix. The design was innovative, and it also solved the need for structural stability, safety, and maximum available parking spaces. 

The project took under 27 months, was completed earlier than scheduled, and came in under budget.

Longest Underground Tunnel

Gotthard Base Tunnel, Switzerland: 35 miles long

Cutting through the Swiss Alps, the Gotthard Base Tunnel is the longest tunnel in the world. The 35-mile tunnel opened to the public in 2016.

The Gotthard pass is the site of other train and vehicle tunnels. However, this tunnel is the first one to create a flat route through the Alps that doesn’t involve any elevation climb. 

About 28 million metric tons of rock were excavated to make the tunnel. The variable composition of the mountains meant that engineers contended with 73 different types of rock, some as hard as granite and others as soft as sand. 

The northern entrance in Erstfeld, Switzerland, is 35 miles away from the southern entrance in Bodio. At its deepest point, the tunnel is 8,040 feet below the surface. Without ventilation, the temperatures at the core of the mountain can reach 115 degrees. 

To put a 35-mile long tunnel into context, compare it to the Hampton Roads Bridge-Tunnel, which is just 3.5 miles long, or the Eisenhower Tunnel through the Colorado Rockies, which is 1.7 miles long.

Deepest Building Foundation

Salesforce Tower, San Francisco: 310 feet below existing grade 

One of the deepest building foundations in the world is below the Salesforce Tower in San Francisco, CA. Completed in 2017, the 1,070-foot skyscraper with 61 stories is supported by a foundation that extends 310 feet below ground.

The foundation design is especially noteworthy because it set new benchmarks in seismic safety. The team of engineers proposed a foundation that made it possible to support the building’s massive weight on the city’s complex soil strata and weak bedrock. It also managed the geotechnical risks, including potential liquefaction and excessive settlement. 

The 310-foot deep foundation of the Salesforce tower surpassed the 282-foot deep foundation for the Shanghai Tower, another skyscraper in an earthquake zone.

The foundation included a 14-foot thick concrete slab that’s about an acre wide. See the 49 million pounds of concrete being poured below.

In contrast to the success of the Salesforce Tower, the foundation design of the Millennium Tower in San Francisco has been rife with problems. The 58-story luxury housing structure opened in 2009. It was built on a foundation of concrete friction piles that are 60 to 90 feet deep, but it didn’t extend as far down as the bedrock. By 2016, the building had sunk 16 inches, and by 2018, it was leaning 14 inches. 

The structure had the telltale signs of foundation problems, including cracks in the building’s basement and surrounding pavement. Each luxury condo owner lost an average of $320,000 in real estate value as a result of the sinking, tilting, and foundation problems. 

Biggest Underground City

Cappadocia, Turkey: 18 stories deep

Underground engineering feats are not limited to modern times. In Cappadocia, Turkey, there was a network of amazing underground cities that date back to 1200 B.C.

A longtime tourist attraction was in Derinkuyu where the public could visit the underground caverns. Now known to be 18 stories deep, it was discovered in 1963 when a Turkish man knocked down a wall in his basement and discovered a tunnel. Since then, more than 600 entrances have been discovered throughout the city. 

In the region, miles of tunnels connect the caves of Derinkuyu to more than 200 other underground cities, all of which were carved into the area’s volcanic rock. 

In 2013, a new subterranean space was discovered in the nearby city of Nevşehir. Early estimates by geophysicists predict this is the largest underground city in the region. It’s about five million square feet in area and extends up to 371 feet below ground, making it 30 percent larger than Derinkuyu. 

Deepest Underground Research Center

China Jinping Underground Laboratory: 7,900 feet deep

Many of today’s leading scientists are able to work on cutting-edge projects by taking their research underground. In the subterranean worlds of the underground research centers, experiments can be shielded from cosmic radiation. 

When sensitive physics experiments are done a mile underground, the cosmic noise is reduced by a factor of 10 million.

The deepest underground research center in the world is below the Jinping Mountains of Sichuan, China. At 7,900 feet deep, researchers explore dark matter. 

One advantage of the building design is that it is accessible via a horizontal access road. Because the underground location is at the center of a mountain, vehicles can easily drive to the site without needing a complicated transportation system to manage the descent. 

In the U.S., the deepest underground laboratory is the Sanford Underground Research Facility in South Dakota, which is 4,889 feet deep. 

This facility is in a former gold mine, and now there are more than 28 active research projects, including the topics of dark matter and neutrino physics research. There’s room for expansion, and the former mine extends 8,000 feet below the surface.

Groundworks is the Leading Foundation Expert