This is maybe not the best data center on Earth, but is certainly one of the coolest under Earth! I stumbled on this article that was written back in November 2008 and although its 'Old News', I thought this was soooo cool!
Pingdom wrote an article on the Pionen
data center, located 100ft underneath the city of Stockholm in Sweeden. This converted nuclear bunker has to be one of the coolest places to go to work every day!
This underground data center has greenhouses, waterfalls, German submarine engines, simulated daylight and can withstand a hit from a hydrogen bomb. It looks like the secret HQ of a James Bond villain.
Read the full article at Pingdom
, and for a 'virtual' walk around, visit Pionen's website; http://www.bahnhof.se/panorama/