Black hole horizons have been a mystery for more than a hundred years. What if they do not exist? Black shells are horizonless black hole mimickers inspired by string theory. The lecture introduces the physics of black shells as well as the observational implications. Why are they black? What is the metric of a rotating black shell? Can EHT or LISA see the difference? Can they act as electric generators powering jets? Theoretical as well as experimental challenges are discussed.