One method to access unconventional, heavy-oil resources is to apply in-situ combustion (ISC) to oxidize in place a small fraction of the hydrocarbon, thereby providing heat and pressure that enhances recovery. ISC is also attractive because it provides the opportunity to upgrade oil in situ by increasing the API gravity and decreasing, for instance, sulfur content. Despite a considerable literature on ISC dynamics, the propagation of a combustion front through porous media has never been visualized directly. We use X-ray computed tomography (CT) to monitor ISC movement, displacement-front shape, and thickness in a 1-m-long "combustion" tube. Temperature-profile history, liquid production, and effluent gas data are also obtained. Tests employ an 8.65API heavy crude oil and representative sand. The general trend of saturation profiles is defined through spatially and temporally varying CT numbers. The role of initial oil and water saturations is examined by packing the combustion tube with either multiple samples with different saturations or by filling it with a uniform sample. Our work quantifies that ISC fronts display instabilities on a fine scale (cm). ISC reactions appear to add to front instability in comparison to inert gas advance. The pressure gradients during ISC appear to influence grain arrangement for loose packing. These grain arrangements cause combustion-front fingering, suggesting that the geomechanical state is relevant to combustion. These new data advance the knowledge base significantly by providing a data set for benchmarking of ISC simulations.