The latest generation of neutrino telescopes uses vast bodies of salt water, fresh water, or ice as the medium for detecting neutrinos. The ANTARES experiment, which recently announced that it has completed construction, is the salt water version, based at the bottom of the Mediterranean Sea.
You can read more about the completion of ANTARES in New Scientist in a report by Rachel Courtland (a former symmetry intern). We also wrote about these neutrino experiments in the very first issue of symmetry magazine back in October 2004.
ANTARES is a real engineering accomplishment because the detectors need to be deep in the water to avoid background signals from cosmic rays. That means they need to be securely attached to the ocean floor, 2500 meters below sea level; able to withstand the high pressure environment; and in positions known well enough that scientists can determine precisely where the neutrinos are.
To get a sense for the challenge of that last problem, imagine attaching a bunch of lightweight cameras to a string of helium balloons that extended hundreds of meters up into the sky. How do you know the precise location of any image the camera records when wind currents are buffeting the balloons and cameras all around? The 12 ANTARES strings of detectors anchored to the sea floor will move by up to 10 or 15 meters at their tops due to ocean currents and so sophisticated techniques are required to figure out just where a neutrino shows up.
However, physicists are confident they have solved this problem, along with the issues of bioluminescent sea creatures that cause false signals, the natural radioactivity of potassium in sea water also causing false signals, and getting power 40 kilometers along the sea floor to the detectors from the coast of France.
The ANTARES telescope has already seen many neutrinos created in the atmosphere but it is really hunting for neutrinos from cosmic sources, and to do that it is looking for particles that come all the way through the Earth. This strange form of neutrino telescope is right at the bottom of the ocean, but even then it is still looking downward.
