Second largest construction after the Great Wall of China
In recent days, it became much more likely that KM3NeT (home page), i.e. Cubic Kilometre Neutrino Telescope, would begin to be built in 2012.
Together with the IceCube Neutrino Observatory in the Antarctica, they would form a global neutrino observatory.
As the name indicates, the volume of the construction filled with photomultiplier tubes is equal to one cubic kilometer. That's quite a volume! The structure should be placed at the bottom of the Mediterranean Sea. Although this would be the second largest construction after the Great Wall of China, probably by the volume, the budget is just €20.8 million, almost 300 times cheaper than the LHC.
(Oops, it's just a small part of the budget: see Anna's correction in the comments.)
They should primarily detect astrophysical neutrinos – with energies between 100 GeV and 0.001 of the GUT scale or so – but if Nature is very generous, it may happen that some more interesting "fundamental" neutrinos related to magnetic monopoles etc. will be seen, too.
Meanwhile, I suppose that lots of fish and other creatures that are able to emit flashes of light etc. will also make the adventures more interesting. I am no expert so you will have to study or ask your nearby experimenter – unless you are one – if you want to know pretty much any detailed information about the "device".
Via PopSci.com and others