A new monoclinic variation of Mg2C3 was synthesized from the elements under high-pressure (HP), high-temperature (HT) conditions. Formation of the new compound, which can be recovered to ambient conditions, was observed in situ using X-ray diffraction with synchrotron radiation. The structural solution was achieved by utilizing accurate theoretical results obtained from ab initio evolutionary structure prediction algorithm USPEX. Like the previously known orthorhombic Pnnm structure (α-Mg2C 3), the new monoclinic C2/m structure (β-Mg2C 3) contains linear C34- chains that are isoelectronic with CO2. Unlike α-Mg2C3, which contains alternating layers of C34- chains oriented in opposite directions, all C34- chains within β-Mg2C3 are nearly aligned along the crystallographic c-axis. Hydrolysis of β-Mg2C3 yields C 3H4, as detected by mass spectrometry, while Raman and NMR measurements show clear C = C stretching near 1200 cm-1 and 13C resonances confirming the presence of the rare allylenide anion.