Brittle materials fail by means of rapid cracks. At their tips, tensile cracks dissipate elastic energy stored in the surrounding material to create newly fractured surfaces, precisely maintaining `energy balance’ by exactly equating the energy flux with dissipation. Using energy balance, fracture mechanics perfectly describes crack motions; accelerating from nucleation to their maximal speed of c<sub>R</sub>, the Rayleigh wave speed. Beyond c<sub>R</sub>, tensile fracture is generally considered to be impossible. By the use of brittle hydrogels, we experimentally demonstrate that a wholly new and different class of tensile cracks that move faster than the shear wave speed, c<sub>s</sub>, exists. The principle of energy balance no longer dictates their dynamics; this new branch of cracks smoothly surpasses c<sub>s</sub> to reach unprecedented speeds that approach the speed of dilatation waves. The transition from `classical’ cracks to these `supershear’ cracks takes place at critical values of applied strains. We, furthermore, show that the values of these, rather moderate (12–14%), critical strains are intimately related to the microscopic material structure. This new mode of tensile fracture represents a fundamental paradigm shift in our understanding of ‘how things break’.