A buried site protects servers well against weather and intrusion. But it complicates everything to do with people and heavy equipment: bringing in a transformer, evacuating a team, extracting smoke, letting firefighters in. These issues cannot be fixed at the end of construction. They are settled in the first sketches.
DOES YOUR SITUATION LOOK LIKE ONE OF THESE?
1. GETTING EQUIPMENT IN, AND OUT
Transformers, generators, switchboards, batteries, chillers: a data center receives equipment weighing several tonnes, which will one day need replacing. In a gallery, the width and height of access routes, turning radii, floor load capacity and room for lifting gear decide what is possible. A structure designed to protect people, like the shelter housing Scaleway's DC4 in Paris, was not drawn for rows of servers. Fitting it out takes all the longer.
The question applies to the whole life cycle: a transformer lowered through a temporary shaft during construction must be able to come out in twenty years.
2. FIRE RISK
A data center concentrates heat sources and combustible materials: batteries, cables, generator fuel. Underground, smoke clears poorly and emergency response takes longer. Several choices are made very early:
What type of batteries, in which room, with what compartmentation
Which fire suppression system, compatible with staff on site
How to extract smoke, and where emergency services come in
Which classified installation rules apply: generators, batteries
Fire and rescue services should be involved early. Their opinion carries weight in the permitting process, and their requirements may change the gallery layout.
3. VENTILATION AND EVACUATION
Ventilation in an underground site does two jobs: removing heat from equipment, and keeping the air safe for people, including during an incident. Evacuation routes, their length, signage and emergency exits must be designed for operations staff and for construction teams, which are often far larger.
4. CONSTRUCTION, THE MOST EXPOSED PHASE
Risks peak during works: heavy lifts in confined spaces, many contractors working side by side, equipment arriving in the halls, electrical works. A dedicated HSE organisation (lifting plans, coordination, site inspections, safety induction) is not a formality: it is what holds the schedule, because an accident stops a site.
Our HSE expert has managed this phase on large industrial and data center projects. He supports the project owner or the general contractor. See also what it takes to convert a quarry or mine and cooling with underground water.
Sources: Scaleway, history of the Lefebvre shelter (DC4); Techniques de l'Ingénieur (5 May 2026); French Environmental Code (classified installations: categories 2910 for combustion, 2925 for batteries); French Labour Code (fire prevention and evacuation of workplaces). 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
Is an underground data center safer?
It is better protected against weather and intrusion. Fire, smoke extraction, evacuation and emergency response, however, are harder and must be addressed at design stage.
Can generators be installed in a gallery?
It is possible but demanding: fuel storage, exhaust, fire risk and maintenance access. Many projects prefer to keep them at the surface. Depending on their power, they are classified installations in France.
When should firefighters be involved in an underground data center project?
From the first sketches. Their requirements on access, smoke extraction and suppression can change the gallery layout.
What are the risks of building a data center underground?
Heavy lifts in confined spaces, contractors working side by side, equipment arriving in the halls and electrical works. A dedicated HSE organisation is essential.