SpringNet Underground is a very well known data center within the industry, as they have received a lot of press attention due to their underground architecture.
The data center is located 85 feet (26~ meters) below the surface in an underground mine, which makes it a safe place for co-location as well as a naturally cooled facility, which lowers the costs of cooling.
The Springnet Underground facility was constructed in 2000 and has a total of 56,500 square feet (5.250~ square meters) with very high ceilings, which has been used to develop a "building in building" concept. It was constructed with high focus on security and is manned 24/7.
Services include racks, foot prints, cages and dedicated rooms (separate buildings in their "building in building concept").
Unfortunately their website isn't very modern, however it does feature interesting details as well as a very cool interactive model of the data center that's definitely worth checking out.
Check out SpringNet Underground (Springfield, Missouri) at Data Center Map.