  The flourishing metal clathrate superhydrides is a class of recently
discovered materials that possess record breaking near-room-temperature
superconductivity at high pressures, because hydrogen atoms behave similarly to
the atomic metallic hydrogen. While series of rare-earth clathrate
superhydrides have been realized, the superconductivity of the first proposed
clathrate calcium superhydride that initiates this major discovery has not been
observed yet and remains of fundamental interest in the field of high-pressure
physics. Here, we report the synthesis of calcium superhydrides from calcium
and ammonia borane precursors with a maximum superconducting temperature of 215
K at 172 GPa, confirmed by the observation of zero resistance through
four-probe electrical transport measurements. Inferred from the synchrotron
X-ray diffraction, together with the consistency of superconducting transition
temperature and equation of states between experiment and theory, sodalite-like
clathrate CaH6 is one of the best candidates for this high-Tc CaHx.
