  A giant peak in the temperature dependence of the screening current is
observed in the ac magnetic response of an ultra-pure YBa$_2$Cu$_3$O$_{6.993}$
crystal in a magnetic field. At H = 2.0 T ({\bf $H$}$||${\bf c}), the screening
current density $J_c(T)$ exhibits a 35-fold rise with 0.5 K increase in
temperature, indicating an abrupt $10^3$-fold collapse in the characteristic
volume of ordered regions in the vortex array. The peak-effect anomaly is most
pronounced for $H<$ 4.0 T, but detectable up to 7.0 T. The temperature
dependence of the equilibrium magnetization exhibits a small discontinuous jump
(for high fields) inside the peak-effect regime, suggesting that the underlying
phase transition is a weak first-order vortex-lattice melting transition.
