ANTARES is a project leading towards the construction and deployment 
of a neutrino telescope in the deep Mediterranean Sea.  The  
telescope  will use an array of photomultiplier tubes to 
detect the Cherenkov light emitted by muons resulting from the 
interaction with matter of high energy neutrinos.  In the vicinity of 
the deployment site the ANTARES collaboration has performed a series 
of in-situ measurements to study the change in light transmission 
through glass surfaces during immersions of several months.  The 
average loss of light transmission is estimated to be only $\sim$~2\% at the
equator of a glass sphere one year after deployment.  It decreases 
with increasing zenith angle, and tends to saturate with time.  The 
transmission loss, therefore, is expected to remain small for the several 
year lifetime of the ANTARES detector whose optical modules are 
oriented downwards.  The measurements were complemented 
by the analysis of the $^{210}$Pb activity profile in sediment cores 
and the study of biofouling on glass plates.  Despite a significant 
sedimentation rate at the site, in the 0.02 -- 0.05~$\rm cm\cdot yr^{-1}$ 
range, the sediments adhere loosely to the glass surfaces and can be 
washed off by water currents.  Further, fouling by deposits of 
light-absorbing particulates is only significant for surfaces facing 
upwards.