  We present a tethered Monte Carlo simulation of the crystallization of hard
spheres. Our method boosts the traditional umbrella sampling to the point of
making practical the study of constrained Gibb's free energies depending on
several crystalline order-parameters. We obtain high-accuracy estimates of the
fluid-crystal coexistence pressure for up to 2916 particles (enough to
accommodate fluid-solid interfaces). We are able to extrapolate to infinite
volume the coexistence pressure (p_{co}=11.5727(10) k_B T/\sigma^3) and the
interfacial free energy (\gamma_{100}=0.636(11) k_B T/\sigma^2).
