Force Oceanique Strategique [FOST] is headquartered at Houilles. This is the strategic command center for the French Naval Nuclear forces. Those 'mushrooms' may be canisters holding antennas that can be deployed ifthe others are destroyed. I would not be surprised if there was a lot of office space underground here.
http://www.netmarine.net/forces/fost/
I found a reference from 2002 stating that "The SSBN command at Houilles (Yvelines) will be shut down and the command shifted to Brest."