 Natural hydrogen has been found in various geologic locations such as oceanic spreading centers, transform faults, passive margins, convergent margins, and interplate settings. It is generated from a variety of sources including alterations in iron-rich rocks, radiolytic reactions with water, and the reaction between water and silica-rich rocks during fracturing. These sources are relatively abundant and provide a potential source for low-cost hydrogen production. This article was authored by Lu Wang, Ji Junjin, Xiao Qin, and others.