An exceptional variability phase occurred in the peculiar F-type hypergiant ρ Cas (Ia+) when the V brightness dimmed by about a full magnitude between June and September 2000. The star recovered from this deep minimum by April 2001. It is the third outburst of ρ Cas on record in the last century. We detect the formation of TiO bands in high-resolution spectra obtained from our long-term monitoring campaigns over the past decade with the Utrecht Echelle Spectrograph on the William Herschel Telescope, and the Sofin spectrograph of the Nordic Optical Telescope (La Palma). Optical and near-IR TiO bandheads, i.e. from the γ -system 0-0 transition at 7050 Å, develop in the summer of 2000. TiO formed before the deep V minimum occurred, and a first analysis shows that during the descending portion of the light curve, the spectrum is a peculiar mixture of a F-type supergiant, with TiO bands that characterize the spectra of M-type stars. This phenomenon has also been observed during the outbursts of 1945-47 and 1985-86. A preliminary analysis of these exceptional spectra is presented, by comparing them with high-resolution optical spectra of the early M-type supergiants μ Cep (Ia) and Betelgeuse (Iab). Central emission is observed above the local continuum level in the split Na D lines. An analysis of ρ Cas' optical emission line spectrum is provided in Lobel 1997, Pulsation and Atmospherical Instability of Luminous F- and G-type Stars, Maastricht: Shaker. We propose the formation of a low-temperature, optically thick, mass shell, caused by instability of the upper atmosphere of this pulsating massive supergiant near the Eddington luminosity limit (Lobel 2001, ApJ, 558, 780). A review is at http://xxx.lanl.gov/format/astro-ph/0108358 This research is supported in part by STScI grant GO-08280.02-97A to the SAO.