"Natural cooling", "zero water", a PUE close to 1: water from a quarry, a mine or a shallow aquifer is one of the most appealing arguments for an underground project. It can genuinely cut cooling costs. But it is also the argument most often presented too quickly, and the one residents and government departments examine most closely.
DOES YOUR SITUATION LOOK LIKE ONE OF THESE?
1. THREE EXAMPLES THAT WORK
Arras. The Décima data center, opened in 2014 in a powder magazine of the citadel, draws water from 40 m down, at around 10 to 11°C. It runs through heat exchangers and is discharged downstream at 17°C at most. A heat pump recovers part of the heat for neighbouring offices, and outside air takes over when it is below 7°C.
Lefdal, Norway. The former olivine mine is cooled with water from a fjord 565 m deep. As the site lies below sea level, it needs no powerful pumps to lift the water. ABB reports a PUE of 1.08 to 1.15 for 5 kW racks.
Iron Mountain, Pennsylvania. The WPA-1 site, in a former limestone mine, uses an underground water reservoir of around 40 hectares.
What they share: an abundant, known and measured resource, built into a system sized for the worst case. None of them relies on a simple pond.
2. WHAT TO CHECK
The real resource. A pond or aquifer is a limited heat sink. Its volume, renewal and temperature in summer and in dry years decide how much heat it can absorb. A hydrogeological study and a multi-year thermal balance are needed, especially for several megawatts.
The type of loop. Three very different situations hide behind the word "water":
Abstraction and discharge: water is drawn and returned warmer, with temperature and quality constraints
Evaporative cooling: it can consume large volumes, especially in summer
The permitting regime. In France, a borehole, groundwater abstraction or warm water discharge may fall under water law, through a declaration or an authorisation depending on volumes and the receiving environment. Heat exchange with the ground also falls under the Mining Code, with a lighter regime for shallow, low-power geothermal systems. These procedures have their own timelines, to be built into the schedule.
Heat does not disappear. The underground helps remove it, it does not eliminate it. A poorly ventilated gallery can even trap heat. And for data centers above 1 MW, a cost-benefit analysis of waste heat recovery is now mandatory in France: see waste heat and district heating.
3. PROMISES TO HANDLE WITH CARE
"Zero water" may be true for server cooling without being true for the whole site. An announced PUE of 1.0, as for the BigNuage project, is a theoretical target that the trade press itself considers out of reach. With worried residents, a broken promise costs more than a cautious figure. Better to publish verifiable indicators: volumes drawn, discharge temperatures, measured PUE.
4. THE METHOD
Measure the resource before sizing, compare at least two scenarios (water and air cooling), identify the applicable permitting regime early, and plan a backup that does not depend on water. This is what our energy expert checks, alongside the overall screening of a quarry or mine.
Sources: Techniques de l'Ingénieur (5 May 2026); Major Vauban Sites Network; ABB press release on Lefdal Mine Datacenter; Iron Mountain, WPA-1 site page; DatacenterDynamics (4 March 2026) and Clubic (18 May 2026) on BigNuage; French Environmental Code, articles L. 214-1 and R. 214-1 (water activities classification); French Mining Code (geothermal energy). 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 cooled with quarry water?
Yes, if the resource is sufficient, renewed and measured, and if abstraction and discharge comply with the applicable rules. The gain depends on a multi-year thermal balance, including summers and dry years.
Is a permit needed to draw water to cool a data center in France?
Depending on volumes and the environment, a borehole, abstraction or warm water discharge may require a declaration or an authorisation under water law. Heat exchange with the ground also falls under the Mining Code.
Does a water-cooled data center consume water?
It depends on the loop: a closed loop consumes little, an abstraction system returns warmer water, and evaporative cooling can consume large volumes.
Is a PUE of 1.0 possible?
Not in practice: a PUE of 1.0 would mean no energy is used outside the servers. As a benchmark, Lefdal, cooled with fjord water, reports between 1.08 and 1.15.