The site looks ideal: several hectares already excavated, few neighbours, a landscape that hides the buildings, rock that stays cool all year. With France's "zero net land take" policy and the race for land, former quarries, mines and caves are attracting more and more developers. Underground data centers already run in France and abroad. But most projects of this kind fail on three points that have nothing to do with the site's appeal.
DOES YOUR SITUATION LOOK LIKE ONE OF THESE?
1. WHAT EXISTING SITES SHOW
Décima, Arras. A 1682 powder magazine in the Vauban citadel, opened as a data center in 2014 (around €2m excl. VAT, 100 m² of white space). Cooled with water drawn from 40 m, at around 10 to 11°C, discharged at 17°C at most.
BigNuage, Saumur area. A 200 MW project announced in May 2026 in galleries dug into tuffeau limestone, at around 12°C all year. Site kept secret, investor not named, works announced for September 2026.
Lefdal, Norway. A former olivine mine with 75 underground halls, operating since 2017, cooled with water from a 565 m deep fjord and powered by hydroelectricity.
Iron Mountain WPA-1, Pennsylvania. A former limestone mine about 67 m underground, 15.5 MW, cooled with an underground water reservoir of around 40 hectares.
These examples are not interchangeable. A military shelter, a powder magazine, tuffeau galleries, a Norwegian mine and an American limestone mine differ in geology, size and access to power. The sites that work are mostly small to mid-sized, or benefit from an exceptional resource, like Lefdal's cold water and hydropower. BigNuage, at 200 MW, would be on a different scale altogether: a project to watch, not yet proof.
2. THE THREE POINTS THAT SINK PROJECTS
Available grid capacity. This is the first and most underestimated constraint. A quarry is rarely next to a substation sized for several tens of megawatts. 5 MW, 20 MW and 200 MW do not involve the same grid operator or the same timelines: above a few tens of MW, the request goes to RTE, at a time when nearly 18 GW of data center requests had already been signed by May 2026. No thermal advantage makes up for a seven-year connection.
The site's legal status. In France a quarry is a classified installation. Its prefectoral permit sets the restoration obligation and the future use of the site, backed by financial guarantees. A mine falls under the Mining Code: after closure, residual subsidence risks may remain and lead to easements. Before discussing a purchase, you need to know where the end-of-operation procedure stands, who is responsible, and whether a data center fits the planned future use and the local zoning plan.
Geology and access. Weakened pillars, fractures, backfill, water ingress: stability must be demonstrated by a geotechnical study. Floors must carry racks, batteries and chillers, and transformers and generators must be brought in, then replaced, over the site's whole life. A gallery designed to extract stone was not designed for that.
3. THE BENEFITS, TO BE PROVEN CASE BY CASE
Land reuse. Reusing an already disturbed site is a real argument, provided the quarry has not become a valuable natural habitat: ponds, colonised rock faces, bats in the galleries. All bat species are protected in France.
Cooling. Rock at a stable temperature and groundwater can reduce the need for mechanical cooling. But the underground does not make heat disappear: it still has to be removed. See our article on cooling with quarry or mine water.
Physical protection. A buried site is less exposed to weather and intrusion. In exchange, fire, ventilation and evacuation are harder to manage: see underground data centers: fire, evacuation, ventilation.
4. THE METHOD: A SCREENING BEFORE ANY INVESTMENT
The classic mistake is to choose a site for its space, then discover that the grid or the geology makes it unworkable. The screening runs the other way round:
2. Site status: prefectoral permit, restoration, future use, liabilities, zoning
3. Geology and access: stability, load-bearing capacity, water ingress, heavy equipment routes
4. Cooling: water resource, closed or open loop, abstraction and discharge rules
5. Environment and neighbours: biodiversity, groundwater, historic pollution, local acceptance
6. Economics: cost of making safe, connecting and fitting out, compared with a standard plot
The first two points rule out most sites within a few weeks, at a fraction of the cost of a full geotechnical study. The guide published by the French Directorate General for Enterprise in November 2025, which lists 63 sites in France favourable to data centers, also puts grid connection and land first among its criteria.
See also our land assessment support, brownfield or greenfield: Nebius's strategy and, for local authorities, what a municipality should check before saying yes.
Sources: Scaleway, history of the Lefebvre shelter (DC4); Techniques de l'Ingénieur, 'Datacenter : voyage au centre de la Terre' (5 May 2026); Major Vauban Sites Network, 'A digital cluster located in a secure area'; DCmag (20 May 2026), Clubic (18 May 2026) and DatacenterDynamics (4 March 2026) on the BigNuage project; ABB press release on Lefdal Mine Datacenter; Iron Mountain, Western Pennsylvania (WPA-1) site page; French Directorate General for Enterprise (DGE), guide on data center siting (28 November 2025); French Environmental Code (classified installations, quarry category 2510); French Mining Code; RTE figures, May 2026. Figures are those published by the operators or the trade press; GridReadiness has not verified them on site. This article is general information, not legal advice.
FREQUENTLY ASKED QUESTIONS
Can a data center be built in a former quarry?
Yes, examples exist in France and abroad. But feasibility depends first on available grid capacity, the site's legal status (restoration, future use) and its geology, before any thermal advantage.
Does an underground data center use less energy?
Not automatically. Cool rock and groundwater can reduce mechanical cooling, but heat must still be removed. The gain has to be shown by a thermal study, not assumed from underground temperatures.
What is the main obstacle to converting a quarry into a data center?
Most often the grid connection: a quarry is rarely close to a substation able to deliver several tens of megawatts within a reasonable timeframe.
What should be checked before buying a former quarry for a data center?
Available power and connection timelines, the quarry's prefectoral permit (restoration, future use, liabilities), zoning compatibility, then geology and water resources.