The first image of a black hole

I will discuss how the first image of a black hole was recently obtained by the Event Horizon Telescope collaboration. In particular, I will describe how modelling the dynamics of the plasma accreting onto the black hole with super-computers using millions of compute hours was essential to generate synthetic black-hole images. I will also illustrate how the comparison between the theoretical images and the observations has allowed us to deduce the presence of a black hole in the nearby galaxy M87 and to extract information about its properties. Finally, I will describe the lessons we have learned about gravity in extreme conditions (the strong-field limit) and alternatives to black holes.