Polycrystalline pyrolytic graphite was compressed in a diamond anvil cell to pressures up to 73 GPa. In situ Raman measurements reveal evidence of a phase transition at 15-20 GPa, consistent with earlier static high-pressure experiments on graphite. The sample was quenched by rapid downloading of the cell to ambient pressure. Micro-Raman investigation of the quenched sample reveals a broad range of previously unobserved intermediate carbon structures, sometimes coexisting with diamond. We also observe the quenching of the 'nanocrystalline' phase of graphite.