Earlier quoted context omitted.
If you're building a relatively slow/cold storage archive datacenter I can think of a lot cheaper, less labor intensive ways to do it than retrofitting an underground bunker... I am assuming they either acquired the bunker for "free" or it has some purpose other than technical (marketing/sales/look how secure your data is!!!).
Not to mention, a bunker in Paris . If it's not speed-critical, it should be off in some mountain or field not an urban center.
And isn't electricity in Paris something like 18 euro cents per kWh? Now your datacenter is eating expensive electricity and requires special expensive cooling systems all year round, because even in the middle of winter there's no easy way to do free-air cooling of an underground bunker.