Recent research to determine the age of the Earth's core used hafnium–tungsten chronometry. The iron-rich metallic core is the most inaccessible part of the Earth, so determining how it has developed is very difficult. Furthermore, the first 500 million years of Earth history are the most obscure; no indigenous rocks from that period have survived the intense meteorite bombardment of the Earth. The development of a new way of determining the timetable for the growth of the core during the very earliest evolution of the Earth therefore represents a major breakthrough.