Bunkers, Mines, and Caverns: The World of Underground Data Centers

extract

Bunkers, Mines, and Caverns: The World of Underground Data Centers Underground facilities promise physical and efficient cooling, but operators face unique engineering and logistic challenge. Underground facilities represent a small but growing niche in the global data center market, driven largely by adaptive reuse of retired mines and bunkers in North America and Northern Europe. Rather than constructing new facilities from scratch, many operators are converting Cold War-era bunkers or disused mines, repurposing hardened, abandoned sites that offer unique advantages for modern deployments. Iron Mountain’s Boyers, Pa., campus, exemplifies this approach. Located more than 200 feet underground, the facility demonstrates how repurposed industrial infrastructure can be engineered to support modern, high-density deployments while meeting the most stringent global compliance standards. “The underground advantage is built on a foundation of resilience, security, and inherent efficiency,” Doug Titzer, senior vice president, design and construction at Iron Mountain, told Data Center Knowledge. By moving hundreds of feet underground, operators gain a natural, impenetrable shield against extreme weather and external threats – a significant benefit for mission-critical workloads in highly regulated sectors, he explained. “At our Boyers facility, we leverage the natural stable ambient temperature of the rock and a 35-acre underground lake to dramatically optimize cooling,” Titzer said. For organizations prioritizing disaster recovery or long-term preservation, this creates a strong alignment between lower power usage effectiveness (PUE) and maximum uptime. Entrance to Iron Mountain’s underground campus in Boyers, Pa., where a former limestone mine has been repurposed for secure data center operations. (Image: Iron Mountain) From Novelty to Proven Niche Underground data centers are no longer a novelty but a proven niche in the data center landscape, according to Max Smolaks, a research analyst for Uptime Intelligence. They continue to appeal for a mix of security, sustainability, and reuse of existing infrastructure. “Underground data centers are a permanent fixture in the industry,” Smolaks told Data Center Knowledge. “There are not many of them, but generally they seem to be working quite well.” European Leaders in Underground Innovation Norway’s Lefdal Mine Data Centers represents one of the more unconventional responses to rising data center energy and sustainability pressures. The facility repurposes a former olivine mine on Norway’s west coast into a large-scale underground operation that uses naturally cool rock temperatures and access to nearby fjord water for efficient cooling, significantly reducing energy demand compared with conventional above-ground data centers. Powered almost entirely by renewable hydroelectric energy, Lefdal is designed to appeal to hyperscalers and enterprise customers seeking lower carbon footprints, higher physical security, and resilience against extreme weather. Swedish internet service provider Bahnhof has become one of Europe’s most visible advocates of underground data centers, using hardened facilities to combine energy efficiency, security, and resilience. Its best-known site, Pionen, is located nearly 100 feet beneath Stockholm in a former civil defense bunker originally built during the Cold War. The facility relies on thick granite walls for physical protection and stable temperatures, while district cooling and renewable energy help reduce operating costs and environmental impact. Inside Bahnhof’s Pionen facility beneath Stockholm – a Cold War-era civil defense bunker reborn as a data center. (Image: Bahnhof) The Underground Advantage: Security, Discretion, and Natural Cooling The primary advantages of underground facilities are physical security and location flexibility, Smolaks explained. Underground facilities are naturally protected from physical threats and weather events, making them ideally suited for "cold storage" (archival data) and high-security government and enterprise workloads. Mines offer large, expandable spaces and can help address “NIMBY” (Not In My Backyard) resistance because they remain largely invisible to surrounding communities. “Nobody’s going to oppose a data center in a mine,” Smolaks said. However, operators still must navigate local permitting, environmental assessments, and community engagement requirements. Siddharth Muzumdar, senior vice president of research at DC Byte, told Data Center Knowledge that the primary appeal of underground data centers lies in passive cooling and extreme physical security. “Subterranean environments offer a constant ambient temperature that can reduce cooling-related energy costs significantly,” he said. Controlled physical access makes such facilities attractive for highly sensitive workloads, and operators can be more selective in site choice, including factors such as proximity to fiber routes and power sources. “Additionally, these facilities provide a ‘pre-built’ structural shell, which can significantly compress the initial construction timeline for the building envelope in a market where time-to-power is now the critical competitive metric,” Muzumdar said. Prime Markets for Underground Development Underground solutions may be most attractive in land-constrained hubs like Singapore, where authorities have explored multi-use underground planning concepts, and in the Nordics, where stable geological conditions and existing mine infrastructure – including Lefdal Mine - facilitate large-scale deployments, Muzumdar explained “Switzerland, with its land constraints, strict planning regimes, and strong demand, is another appealing market,” he said. The SubTropolis Technology Center is located in Hunt Midwest's underground limestone mine complex in Kansas City, Missouri. (Image: Hunt Midwest) The Complexity Trade-Off While subterranean facilities are high-performance assets, they aren't a one-size-fits-all solution, Titzer noted. The primary tradeoff is complexity. “Development requires specialized engineering to navigate site-specific geological constraints and more rigid expansion paths compared with traditional greenfield builds,” Titzer said. Additionally, logistics – ranging from heavy equipment transport to long-term infrastructure upgrades – require more sophisticated planning in a subterranean setting. “Ultimately, these facilities are most viable when the mission-critical nature of the data justifies the engineering investment required to protect it,” Titzer said. Operational Challenges: Scalability, Heat, and Logistics The drawbacks of underground data centers are largely operational, Smolaks said. Humidity control, ventilation, and temperature management are more complex underground, and retrofitting older structures for modern cooling demands can be costly. Logistics pose another challenge, as transporting equipment and staff to remote or enclosed sites is more difficult than on conventional campuses. Still, Smolaks pointed out that many underground facilities remain under the same ownership for years, suggesting they are commercially viable. “If there were major problems, they would be a hot potato,” he said. “The fact that the same operators are still running them tells you they work.” Site Selection Complexities Matt Kennedy, global head of client transformation at IDA Ireland, said selecting a location for an underground data center is a highly intricate process requiring careful evaluation of several critical factors. “The structural stability of the site, its proximity to reliable power sources, and considerations for environmental safety are paramount,” he explained. For example, sites near geological faults or water sources can pose heightened risks, such as increased earthquake susceptibility or water ingress. “Without robust systems in place, the risk of damage to equipment rises considerably,” Kennedy said. Vertical Scalability Challenges From Muzumdar’s perspective, the most significant drawback is the lack of "vertical scalability." Operators are physically constrained by the existing rock geometry and cannot easily add floors to accommodate AI-driven density spikes. “Furthermore, while the initial shell exists, the specialized Capex required for moisture control, radon mitigation, and complex vertical fiber routing can often offset the savings gained from forgoing a traditional building,” he said. Managing heat rejection is the primary technical hurdle, as the surrounding rock acts as an insulator rather than a heat sink, requiring sophisticated ventilation or liquid cooling loops to move thermal loads to the surface. “Other considerations include fire suppression and smoke extraction systems,” Muzumdar added. Mastering the Underground Environment Technical success underground depends on mastering three variables: cooling, connectivity, and airflow, Titzer said. While stable rock temperatures provide a "head start" on cooling, high-density environments still require robust, custom-engineered ventilation systems to manage heat load effectively. “Furthermore, we must ensure that network access and carrier diversity match the high-speed performance of any Tier III above-ground campus,” he said. Finally, maintaining rigorous compliance and monitoring standards in a repurposed mine requires a specialized operational mindset. “The environment is unique, but the security and performance expectations remain absolute,” Titzer said.