Shear (S) waves moving through the inner core travel at about fifty percent of the velocity expected for most iron-rich alloys.[18] Considering the core's composition is widely believed to be an alloy of crystalline iron with a small amount of nickel, this seismographic anomaly points to another substance's existence within the region. One theory postulates that such a phenomenon is the result of various light elements, including carbon, in the core.[18] In fact, studies have utilised diamond anvil cells to replicate the conditions in the Earth's core, the results of which indicate that iron carbide (Fe7C3) matches the inner core's sound and density velocities considering its temperature and pressure profile. Hence, the iron carbide model could serve as evidence that the core holds as much as 67% of the Earth's carbon.[19] Furthermore, another study found that carbon dissolved in iron and formed a stable phase with the same Fe7C3 composition—albeit with a different structure than the one previously mentioned.[20] Hence, although the amount of carbon potentially stored in the Earth's core is not known, recent research indicates that the presence of iron carbides could be consistent with geophysical observations.