We report the largest mass-loss rate of ~5 % of the solar mass per year, directly observed in any stellar object so far during the recent outburst of the Yellow Hypergiant ρ Cas. In the fall of 2000, the enigmatic cool luminous supergiant dimmed by more than a full visual magnitude, thereby changing its spectral type form early F- to early M-type. Our spectral monitoring reveals that the effective temperature decreased by more than 3000 K during the event, from above 7000 K to below 4000 K in less than ~200 days. It is the third outburst of ρ Cas on record in the last century. The optical spectrum becomes comparable to that of the red supergiant Betelgeuse, and reveals strongly blue-shifted molecular absorption bands of titanium-oxide (TiO). We determine from the newly formed TiO bands a gas mass-loss rate of the same order of magnitude as has been proposed for the giant outbursts of the Luminous Blue Variable η Carinae. The outburst of ρ Cas produces an outward propagating circumstellar shock wave, driven by recombination of hydrogen gas in its wake, resulting in a tremendous cooling of the entire outer atmosphere. Over the past 18 months since the outburst we observe a very prominent inverse P Cygni profile in Balmer Hα . Strong emission in this line has not before been observed in ρ Cas over this long period of time. The optical spectroscopic monitoring signals an unusal strong collapse of the upper Hα atmosphere, which we also observed in the months before the outburst event. The recent observations may therefore signal that a new and stronger outburst of ρ Cas is imminent. This reseach has been supported in part by a Space Telescope Science Institute grant to the Smithsonian Astrophysical Observatory.