  We report on the study of interstellar extinction across the Tarantula nebula
(30 Doradus), in the Large Magellanic Cloud, using observations from the Hubble
Tarantula Treasury Project in the 0.3 - 1.6 micron range. The considerable and
patchy extinction inside the nebula causes about 3500 red clump stars to be
scattered along the reddening vector in the colour-magnitude diagrams, thereby
allowing an accurate determination of the reddening slope in all bands. The
measured slope of the reddening vector is remarkably steeper in all bands than
in the the Galactic diffuse interstellar medium. At optical wavelengths, the
larger ratio of total-to-selective extinction, namely Rv = 4.5 +/- 0.2, implies
the presence of a grey component in the extinction law, due to a larger
fraction of large grains. The extra large grains are most likely ices from
supernova ejecta and will significantly alter the extinction properties of the
region until they sublimate in 50 - 100 Myr. We discuss the implications of
this extinction law for the Tarantula nebula and in general for regions of
massive star formation in galaxies. Our results suggest that fluxes of strongly
star forming regions are likely to be underestimated by a factor of about 2 in
the optical.
