Abstract: The mechanism of many earthquake precursors is related to fluids deep inside the Earth. In this paper, recent studies on the status of fluids inside the earth are reviewed. The quantity, existence and quality of the water, pore pressure and interconnection characteristics are discussed with regard to indirect and direct evidence from explorations with super-deep drill holes. According to recent results, water under high temperature and pressure within the earth is in a super-critical state and has many special characteristics. As a result, the liquids not only become the propagation medium for strong earthquake precursors and the influential factor in the process of earthquake source rupture, but are also a medium that directly takes part in the dynamic quake process.