  The vortex system in a high-T_c superconductor has been studied numerically
using the mapping to 2D bosons and the path-integral Monte Carlo method. We
find a single first-order transition from an Abrikosov lattice to an entangled
vortex liquid. The transition is characterized by an entropy jump dS = 0.4 k_B
per vortex and layer (parameters for YBCO) and a Lindemann number c_L = 0.25.
The increase in density at melting is given by d\rho = 6.0*10^{-4} /
\lambda(T)^2. The vortex liquid corresponds to a bosonic superfluid, with
\rho_s = \rho even in the limit \lambda -> \infty.
