Cardiac fibrillation is driven by nonlinear waves of excitation, which evolve rapidly and in a chaotic manner through the heart muscle and trigger asynchronous, irregular contractions. The visualization of these highly dynamic three-dimensional wave phenomena within the volume of the heart muscle has remained a major scientific challenge. Using high-resolution 4D ultrasound ex vivo, we showed that it is possible to analyze the rapid muscle deformations during fibrillation and to identify mechanical filament-like phase singularities within the contracting, fibrillating ventricular wall, which like fingerprints of electrical vortex filaments evolve through the heart wall and presumably indicate the core regions of three-dimensional electrical scroll waves. Simultaneous tri-modal and panoramic fluorescence imaging of the deforming heart surface shows that electrical action potential and calcium spiral vortices evolve largely congruently across the heart surface during fibrillation and create vortex-like mechanical deformations and mechanical rotor patterns, which similarly rotate and whose core regions co-exist and co-localize with the core regions or phase singularities of the electrical action potential or calcium vortex rotors. Our 3D imaging data suggests that cardiac fibrillation can be characterized through mechanical phase singularities and contraction vortex filaments, and that ultrasound can, similar to fluorescence imaging, provide highly detailed maps of complex arrhythmias. Prerequisite for the simultaneous imaging of electrical and mechanical activity and the combined use of fluorescence imaging and ultrasound was the advancement of optical and computer vision techniques to successfully compensate so-called motion artifacts. Moreover, the analysis of the spatio-temporal elastic patterns can be enhanced employing a model-assisted imaging approach, which could ultimately help to estimate the electrical activity that had caused the fibrillatory deformations. We expect that our findings will significantly enhance the understanding of cardiac fibrillation, and will stimulate the development of novel instrumentation for arrhythmia imaging and may lead to novel diagnostic and therapeutic approaches.