The combustion of hydrogen produces only water without releasing CO2, and thus hydrogen is considered to be the most environmentally friendly form of green energy. As a result, the potential of naturally-occurring hydrogen as a renewable carbon-free energy source has attracted a research boom amongst global communities in energy, science and technology. The "Hunt for natural hydrogen heats up" was selected as one of the "2023 breakthroughs of the year" in the journal Science. Direct extraction of natural hydrogen with economic values stored in economic quantities in geological bodies is probably the cheapest approach to produce hydrogen. Therefore, natural hydrogen is often called "gold hydrogen". The geographical and geological distribution of natural hydrogen is summarized in this study, and its mechanisms of formation are discussed, with a view to help assess the resource potential of natural hydrogen. The study indicates that natural hydrogen is widely distributed on Earth. Natural hydrogen seeps are mainly located at the structurally active belts such as mid-oceanic ridges, ophiolite belts in the convergent margins of the plates, magmatic active zones, hot spring areas, and structurally stable zones such as Precambrian continental basements. Hydrogen has been discovered in natural gas from Chinese petroliferous basins such as Qaidam, Subei, Songliao, Bohai Bay, Ordos, and Tarim. However, little attention has been paid to it in the past. Hydrogen in natural gas is typically present in low concentrations, and therefore there are limited studies on the geological formation mechanisms of hydrogen, which has aroused extensive attention and conjecture due to the diversification of the formation environment. The reaction of water with iron-bearing minerals, represented by serpentization, is thought to be the most important mechanism of hydrogen formation, and the decomposition of water by radiation during radioactive decay is also considered as a possible mechanism for significant hydrogen generation. The Earth's core and mantle contain both primordial hydrogen and hydrogen produced by different chemical reactions, and hydrogen generated by both these deep sources can reach the surface through degassing. Moreover, the decomposition of organic matter and biological activity are also believed to be potential approaches to generate hydrogen. These mechanisms have all existed since primitive Earth time and will continue in the future. The processes such as the water-rock reactions are reproducible in nature, therefore, natural hydrogen is considered as a renewable energy. The main concern of the natural hydrogen industry is the availability of sufficient natural hydrogen resource. The annual natural hydrogen fluxes from mid-oceanic ridges, water-rock reactions and decomposition of water by radiation are commonly in the magnitude of 10(10)-10(13)g according to different estimates. However, hydrogen diffuses easily due to its small molecular size and is chemically reactive. Therefore, natural hydrogen in geological bodies is commonly too dispersed to extract commercially. Several hydrogen research institutions and companies have been established abroad in recent years, and attempts have been made to explore for natural hydrogen. However, exploration is still in the infancy. Surface hydrogen seeps may indicate the potential for finding large-scale hydrogen gas pools underground. The exploration and exploitation of natural hydrogen resource have broad prospects, and it is necessary to further understand and evaluate the occurrence of natural hydrogen resources and develop hydrogen detection technology and equipment. This will not only help with understanding generation, migration and enrichment mechanisms of natural hydrogen, but also reduce the risk in hydrogen exploration and promote the development of a hydrogen energy economy.
