A team of scientists has developed four-dimensional (the three dimensions of space plus the fourth dimension of time) atomic electron tomography. Tomography is a technique for creating images of cross sections of an object using X-rays or ultrasound. The technique directly images the dynamics of structural changes at the atomic scale during nucleation. Nucleation is the creation of structure in a vapor, solution, or liquid. The scientists found that the nuclei came in a broad range of shapes and sizes and possess a diffuse interface surrounding a stable core. Their observations challenge the long-held classical nucleation theory that posits nucleation begins with the formation of perfectly spherical nuclei that grow after they reach a certain critical size.