Optical background measurement in a potential site for the NEMO KM undersea neutrino telescope

We describe the measurement of the deep sea optical background in some sites south of Capo Passero, Sicily. A continuous flux of about 440 photons cm -2 s -1 in the wavelength interval 440-550 nm is estimated due to the decays of the 40K contained in the sea water. Bioluminescence light bursts are also observed and an example of time evolution is reported. All the measurements were accomplished using a deep sea module also described in this article