  Disordered systems like liquids, gels, glasses, or granular materials are not
only ubiquitous in daily life and in industrial applications but they are also
crucial for the mechanical stability of cells or the transport of chemical and
biological agents in living organisms. Despite the importance of these systems,
their microscopic structure is understood only on a rudimentary level, thus in
stark contrast to the case of gases and crystals. Since scattering experiments
and analytical calculations usually give only structural information that is
spherically averaged, the three dimensional (3D) structure of disordered
systems is basically unknown. Here we introduce a simple method that allows to
probe the 3D structure of such systems. Using computer simulations we find that
hard-sphere-like liquids have on intermediate and large scales an intricate
structural order given by alternating layers with icosahedral and dodecahedral
symmetries, while open network liquids like silica have a structural order with
tetrahedral symmetry. These results show that liquids have a highly non-trivial
3D structure and that this structural information is encoded in non-standard
correlation functions.
