Iron Mountain has converted an old mine in Pennsylvania to a computing facility, part of which includes an experimental energy-efficient data center that uses geothermal conditions to improve cooling.
ComputerWorld wrote an article about their tour of the facility, including Room 48 where the experimental data center is housed. The power distribution transformers and the air conditioning units are outside the data center, rather than inside it. It also relies on the extreme pressure differential between the hot and cold aisle to move the air, making the data center very quiet (in contrast to the usual data center).
The data center in Room 48 operates at between 70 and 72 degrees Fahrenheit, and operates 200 watts per square foot (more than 50% above the usual 125 watts per square foot).
This data center is truly remarkable.