Most known exoplanets orbit close to their host stars, where they are exposed to tremendous amounts of irradiation. This leads to atmospheric expansion and hydrodynamical thermal escape (or ``evaporation"). This phenomenon could play a critical role in the evolution of low-mass exoplanets and in shaping the observed exoplanet population. Ultraviolet observations made with the Hubble Space Telescope have enabled the detection of atmospheric evaporation on hot gas giants. In this seminar, I will focus on my team's recent and surprising detection of a huge cloud of hydrogen escaping from a gently-irradiated Neptune-mass planet. I will discuss the exciting perspectives opened by this new result, especially for the future atmospheric characterisation of terrestrial exoplanets.