In the ongoing megagrant project we aim to conduct experimental studies on the earth interior and its formation processes combined with high pressure experiments, synchrotron X-ray radiation, and theoretical studies using ab-initio calculations. We have conducted broad topics related to earth’s interior from mantle to the core. Here we present our recent results on the Earth’s core. We have determined the sound velocity and density of hcp-iron, a major constituent of the inner core. We have successfully measured sound velocity and density at the Earth’s core conditions up to 170 GPa and 3000 K, the highest pressure and temperature ever achieved by synchrotron X-ray inelastic scattering spectroscopy and X-ray diffraction combined with a double sided laser heating diamond anvil cell. Our results revealed that the sound velocity of hcp-iron extrapolated to the inner boundary (ICB) conditions, 330 GPa and 5500 K, has a larger compressional velocity and density compared to the those of the PREM (Preliminary Reference Earth Model) inner core at ICB. Thus, inner core contains light elements that reduce both compressional velocity and density of pure hcp-iron.