##C/2014 UN271## (Bernardinelli-Bernstein) is a comet incoming from the Oort cloud which is remarkable in having the brightest (and presumably largest) nucleus of any well-measured comet, and having been discovered at heliocentric distance ##r_h \approx 29 \;\operatorname{au}##, farther than any Oort-cloud member. We describe in this work the discovery process and observations, and the properties that can be inferred from images recorded until the first reports of activity in June ##2021.## The orbit has ## i = 95°##, with perihelion of ##10.97\;\operatorname{au}## to be reached in ##2031,## and previous aphelion at ##40, 400 \pm 260\;\operatorname{au}.## Backwards integration of the orbit under a standard Galactic tidal model and known stellar encounters suggests this is a pristine new comet, with a perihelion of ##q \approx 18\;\operatorname{au}## on its previous perihelion passage ##3.5\;\operatorname{Myr}## ago. The photometric data show an unresolved nucleus with absolute magnitude ##H_r = 8.0,## colors that are typical of comet nuclei or Damocloids, and no secular trend as it traversed the range ##34–23\;\operatorname{au}##. For ##r##-band geometric albedo ##p_r##, this implies a diameter of ##150\,(p_r/0.04)−0.5 \;\operatorname{km}.## There is strong evidence of brightness fluctuations at ##\pm 0.2## mag level, but no rotation period can be discerned. A coma, nominally consistent with a "stationary" ##1/\rho## surface-brightness distribution, grew in scattering cross-section at an exponential rate from ##Afρ \approx 1\; m## to ##\approx 150\; m## as the comet approached from ##28## to ##20\;\operatorname{au}.## The activity rate is consistent with a very simple model of sublimation of a surface species in radiative equilibrium with the Sun. The inferred enthalpy of sublimation matches those of ##CO_2## and ##NH_3##. More-volatile species such as ##N_2##, ##CH_4##, and ##CO## must be far less abundant on the sublimating surfaces.
