The status of the low-temperature phase diagram of hydrogen at the turn ofthe century

Hydrogen. is the simplest element in nature. This simplicity in the atomic state is often assumed to hold also for its condensed phases. Nevertheless; experiments carried out during tile past 15 years of the XXth century have shown that this picture is not necessarily a faithful one. Several differe nt low-temperature solid phases have been identified, in contrast with the simplicity idea. These exhibit outstanding features like pressure-independe nt phonon bands, large isotope effects, and strong infrared activity. In th is paper I will give an overview of the current understanding of the low-te mperature region of the phase diagram of hydrogen, as emerges from a fruitf ul cooperative action between diamond anvil cell experiments and first-prin ciples theoretical calculations.