For the last 25 years, the 21 cm line has been used productively to investigate the large-scale structure of the Universe, its peculiar velocity field and the measurement of cosmic paramaters. In February 2005 a blind HI survey of 7000 square degreees of the high latitude sky was started with the Arecibo radio telescope, using the 7-beam feed L-band feed array (ALFA). Known as the Arecibo Legacy Fast ALFA (ALFALFA) Survey, the program will produce a census of HI-bearing objects over a cosmologically significant volume of the local
Universe. When completed, well in excess of 20,000 HI sources will be detected.With respect to previous blind HI surveys, ALFALFA offers an improvement of about one order of magnitude in both sensitivity and areal resolution. As of late 2007, ~ 1/2 of the survey observations and ~ 20% of the source extraction are completed. Preliminary results of ALFALFA will be presented, with emphasis on the distribution of cold gas bearing galaxies and the possible existence of optically "dark" halos.