  The insulator-metal transition in hydrogen is one of the most outstanding
problems in condensed matter physics. The high-pressure metallic phase is now
predicted to be liquid atomic from T=0 K to very high temperatures. We have
conducted measurements of optical properties of hot dense hydrogen in the
region of 1.1-1.7 Mbar and up to 2200 K. We observe a first-order phase
transition accompanied by changes in transmittance and reflectance
characteristic of a metal. The phase line of this transition has a negative
slope in agreement with theories of the so-called plasma phase transition.
