Amorphous calcium carbonate (ACC) is a metastable hydrated solid that has received great attention as a precursor in calcium carbonate crystallization in both synthetic and biological systems. In particular, the atomic structure of ACC is a matter of ongoing discussion. Some studies have pointed out similarities between the local structure of amorphous calcium carbonate and its crystalline counterparts, whereas others suggested no resemblance to any known crystalline form. Despite the large number of studies, few structural aspects have been described beyond the first Ca–O distance and coordination number. Specifically, the role of carbonate ions and water molecules in the amorphous network is poorly understood. Here we address this issue using neutron and X-ray total scattering in combination with molecular modeling on a set of well-defined synthetic CaCO3·nH2O samples, synthesized by rapid mixing of CaCl2 and Na2CO3 solutions and with two levels of hydration, n = 1.1 and n = 0.5. An atomistic model of A...