This proposal requests partial funding to construct a beamline for high-intensity x-ray diffraction analysis of matter under extremely high pressures in the megabar range. The beamline will make use of the intense radiation generated by the interaction of rapidly accelerated electrons with superconducting wiggler magnets at the National Synchrotron Light Source at Brookhaven National Laboratory. The facility will allow diamond anvil cells, in which a very small sample can be subjected to intense pressure and temperature, to be connected to a finely focused x-ray source so that the material structure can be characterized. Use of the beamline radiation in conjunction with ultra-high pressure devices will allow the study of matter under conditions comparable to those encountered in planetary interiors.