We have discovered absorption of the 3 K microwave background radiation by ultracold CO gas in the Boomerang Nebula, a bipolar reflection nebula illuminated by a star that has recently evolved off the asymptotic giant branch (AGB). During the AGB phase, stars with main-sequence masses of 1-8 M eject large amounts of matter, affecting their subsequent evolution as well as the chemical and dynamical evolution of the Galaxy. Our new observations of CO and ¹³CO millimeter-wave lines toward the Boomerang Nebula show it to be quite extreme and perhaps unique in its mass-ejection properties. We find that it has been losing mass through a fast (164 km s^-1) molecular wind at a prodigious rate of 10^-3 M yr^-1 (a factor of about 10 larger than the highest rates seen in AGB/post-AGB objects until now) for at least approximately 1500 yr. This wind contains ultracold gas at temperatures below the microwave background temperature, making the Boomerang Nebula the coldest place in the universe found so far (excluding laboratories), and confirming an earlier prediction of the existence of such envelopes. The ¹²C/¹³C ratio is rather low (5), close to the lowest value attainable (3) through equilibrium CNO-cycle nucleosynthesis. The mechanical wind momentum (dM/dt × V_exp) in the Boomerang Nebula exceeds the total radiative momentum (L_*/c) by a factor greater than 10⁴. The data also show the presence of an inner shell, expanding at 35 km s^-1, which may have resulted from the ejection of a common envelope by a central binary star.