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Calcium carbonates play important role in Earth's deep carbon cycle. They have been studied extensively at conditions of the upper mantle, while little is known about their fate at higher pressures. In this work, the Ca-C-O system is studied under high pressure (122 GPa) and high temperature (3000 K), close to conditions of Earth's core-mantle boundary. High-pressure single-crystal X-ray diffraction was conducted at the ID15b and ID27 beamlines of the ESRF, demonstrating synthesis of two novel calcium carbonates. CaC2O5 is the first example of a carbonate based on a tetrahedral framework, while the CaCO3 elucidates questions of the crystal chemistry and chirality of CaCO3 at deep-Earth conditions. This study shows that Ca-C-O system under high pressure is more complex than previously anticipated, featuring different stoichiometries, conformations and connectivity of the CO4 tetrahedra.