Metal closo-borates and their derivatives have shown promise in several fields of application from cancer therapy to solidstate electrolytes partly owing to their stability in aqueous solutions and high thermal stability. We report the synthesis and structural analysis of α- and β-CaB10H10, which are structurally and energetically similar, both showing a tetrahedral coordination of Ca2+ to four closo-borate cages. The main distinctions between the α- and β- polymorph are found in the crystal system (monoclinic or orthorhombic), topology (wurtzite or cag), and the degree of displacement of Ca2+ from the center of the coordination tetrahedron. Neutron vibrational spectroscopy measurements further revealed distinct perturbations in the cation−anion interactions arising from the different crystal structures. We also synthesized and structurally investigated five stoichiometric hydrates, CaB10H10· xH2O, x = 1, 4, 5, 6, and 7, and discovered an order−disorder polymorphic transition, α- to β-CaB10H10·6H2O. The hydrates reveal a rich structural diversity with ordered structures, CaB10H10·xH2O, x = 1, 4, 5, 6, and 7, as well as disordered structures, x = 6 and 8. The latter allow for a continuum of compositions within 7−8 molecules of crystal water. The DFT-optimized experimental crystal structures reveal complex networks of three types of hydrogen interactions: dihydrogen bonds, B−Hδ−···+δH−O; hydrogen− hydrogen interactions, B−H···H−B; and hydrogen bonds, O−Hδ+···−δO−H. A rather short B−H···H−B (2.14 Å) interaction is observed for CaB10H10·5H2O, which is locally stabilized by four hydrogen bonds.