We propose the first high P/T neutron diffraction measurements on goethite (FeOOH), both the ambient pressure alpha- as well as the high-pressure epsilon-phase, in the P/T range of 0-10 GPa and 300-500 K. Goethite is the most common ferric oxyhydroxide on Earth and possibly on Mars. The pressure dependence of its structure as a function of P and T has been studied by synchrotron x-ray diffraction which obviously could not detect hydrogen nor adress the issue of magnetic ordering. The experiments proposed here will allow locating the hydrogen/deuterium positions, revealing the behaviour of the H-bond as a function of strong compression, and will determine the pressure dependence of the Néel temperature. The results will contribute to a better understanding of H-bonded minerals as well as our understanding of magnetic ordering in fundamental iron-rich solids