St. Ghislain, Belgium
Image via Bing Maps: binged.it/zQnjaL (Google Maps is out of date for this location)
Estimated server count: 296,960
Google chose Saint-Ghislain in 2007 and began operation in late 2008. The Google Maps imagery currently shows only empty space.
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Google's own page about this data center: google.com/datacenters/st-ghislain