Third-generation synchrotron radiation sources are currently becoming operational. These powerful x-ray radiation sources will be critical in advancing research in key areas of science, engineering, and medicine. Efficient utilization of these sources requires the development of critical beamline optical components which can withstand their very intense beams without significant distortion. In this paper we will discuss the applications of an innovative, low-cost, castable form of SiC as a monolithic cooled mirror substrate for use on high energy synchrotron beamlines. The superior bulk material properties of SiC--excellent thermal conductivity, a very low coefficient of thermal expansion, excellent specific stiffness and non-reactive with typical coolants--are well known. In addition to the superior bulk material properties, this high purity form of SiC has a number of other desirable characteristics which make it particularly well suited for this application: 1) it can be fabricated with complex, internal cooling channels in a monolithic fashion; 2) it has been demonstrated to provide the excellent surface figures and surface finishes required for x-ray optics applications; and 3) the castable SiC can be manufactures in a very low cost manner, particularly in high volumes. Overall, the innovative SiC mirror substrate discussed promises to offer improved performance, significantly reduced cost, and reduced risk compared to present approaches.