Located in Stockholm, Sweden, it was used by Sweden's Civil Defense in 1970 but was decommissioned and converted into a data center in 2008 by Internet Service Provider Bahnhof.

The conversion took two years to blast out 141,000 cubic feet it needed to fit all their backup generators and server racks.

Wikileaks actually used this space as its data center.

It housed the servers for the website here because it has one of the world's strongest laws to protect confidential source-journalist relationships.

But while it is underground, it doesn't look anything like a data center.

It's got stuff like this:

It also has its own greenhouses, simulated daylight, a huge saltwater fish tank and two huge submarine engines that act as backup generators.

Check out more pictures below: