Simple molecules as water, ammonia and methane are known to be the main constituents of the ice-giants (Uranus and Neptune) of icy moons and of several Neptune-like exoplanets. Knowing how these simple molecules behave at extreme conditions of pressure and temperature is important to build reliable models for planetary interiors. These molecular systems which are simple from the chemical point of view, show an extraordinary complexity from the physics point of view. Mixtures of water and other simple molecules also showcase peculiar behaviors and are an ideal playground for gaining a clearer understanding of the physics of hydrogen bonds and of hydrophobic interactions. In this seminar I will present some of our results by High Pressure (HP) Quasi-Elastic Neutron Scattering (QENS). I will introduce the technique and the HP setups and show how QENS is a powerful technique to directly probe the hydrogen dynamics of molecular systems under extreme conditions.