  The NEMO (NEutrino Mediterranean Observatory) Collaboration has been carrying
out since 1998 an evaluation programme of deep sea sites suitable for the
construction of the future Mediterranean km3 Cerenkov neutrino telescope. We
investigated the seawater optical and oceanographic properties of several deep
sea marine areas close to the Italian Coast. Inherent optical properties (light
absorption and attenuation coefficients) have been measured as a function of
depth using an experimental apparatus equipped with standard oceanographic
probes and the commercial transmissometer AC9 manufactured by WETLabs. This
paper reports on the visible light absorption and attenuation coefficients
measured in deep seawater of a marine region located in the Southern Ionian
Sea, 60-100 km SE of Capo Passero (Sicily). Data show that blue light
absorption coefficient is about 0.015 1/m (corresponding to an absorption
length of 67 m) close to the one of optically pure water and it doe not show
seasonal variation.
