Although the presence of carbon in the Earths core is well-constrained, recent studies suggest large inventories of carbon could be stored in this region. Shear (S) waves moving through the inner core travel at about fifty percent of the velocity expected for most iron-rich alloys. Because the cores composition is believed to be an alloy of crystalline iron and a small amount of nickel, this seismic anomaly indicates the presence of light elements, including carbon, in the core. In fact, studies using diamond anvil cells to replicate the conditions in the Earths core indicate that iron carbide (FeC) matches the inner cores wave speed and density. Therefore, the iron carbide model could serve as an evidence that the core holds as much as 67% of the Earths carbon. Furthermore, another study found that in the pressure and temperature condition of the Earths inner core, carbon dissolved in iron and formed a stable phase with the same FeC composition—albeit with a different structure from the one previously mentioned. In summary, although the amount of carbon potentially stored in the Earth's core is not known, recent studies indicate that the presence of iron carbides can explain some of the geophysical observations.