In all likelihood, you wouldn't envision a circular web of cables strung with 12,000 beach ball-like detectors, each cable about twice as tall as New York's Empire State Building, anchored two miles deep on the floor of the Mediterranean Sea.
The telescopic array just described is called the Cubic Kilometer Neutrino Telescope, or KM3NeT for short. When operational, its primary mission will be to detect high energy, elusive, subatomic particles -- neutrinos -- and map their stellar sources.
Neutrinos move so fast, and are so tiny, that they can pass through almost anything. But traveling at near the speed of light, they occasionally strike atoms and leave tiny residual signatures of their presence, called Cherenkov radiation, which manifests as a cone of blue light. (That's why nuclear reactors glow blue.) By building KM3NeT at the bottom of the Mediterranean Sea, submersed in saltwater -- a medium much denser than air -- astrophysicists increase the chance that neutrino collisions will occur, and thus that the thousands of sensors affixed to the telescope will detect them.
As you've likely surmised, KM3NeT is undeniably different compared to land-based telescopes. Most fundamentally, while optical and radio telescopes train their gaze to the heavens above, KM3NeT actually focuses downward. It's arranged as such in order to avoid detecting particles with more local origins. Earth's atmosphere is already brimming with neutrinos, but KM3NeT isn't interested in those. Thus, the telescope uses the Earth as a sort of shield to filter them out.
At an estimated cost upwards of 250 million Euros, the project is a tad pricey. Luckily, seven European countries -- France, the Netherlands, Germany, Greece, Romania, Spain, and Italy -- are pitching in.
Construction has recently embarked on research structures in the Mediterranean, off the shores of Toulon, France; Porto Palo di Capo Passero, Italy; and Pylos, Greece. By Spring 2013, construction will be underway on the thousands of calibration and detection units required for the array. Full, on-site construction is slated to begin in 2014.
When completed, KM3NeT will train its gaze toward the galactic center, where it will hopefully detect neutrinos from pulsars, the environs of black holes, and some of the universe's earliest supernovae. Astrophysicists hope that these long-distance cosmic messengers will reveal fascinating stories about cataclysmic events, and potentially even inform us on the nature of dark matter.
(Images: 1) VLT by G. Hüdepohl/ESO via Wikimedia Commons 2 & 3) KM3NeT via Marco Kraan/Propiety KM3NeT Consortium)