According to a story in HPCwire, the number two supercomputer on the Green500 list is a little different from all the other honorees - it's the only one that is air-cooled.  

Cambridge University's Wilkes - as the supercomputer has been named - is based on 128 Dell T620 servers, which provide greater energy efficiencies.

Capable of 240 teraflops LINPACK, Wilkes  is a part of Cambridge's "Square Kilometer Array (SKA) Open Architecture Lab," which is participating in a multinational collaboration to build the world's largest radio telescope.  As was mentioned in his SC13 video, it's his work with SKA that has Paul Calleja's son convinced dad is looking for aliens in outer space!

You can learn more about Cambridge's air-cooled supercomputer at HPCwire.