Hydrogen greatly affects the properties of many materials. It is thought that most of the hydrogen in modern Earth is in water molecules, many of which are found in water-bearing minerals. It is therefore crucial to understand the stability and circulation of such hydrous minerals in Earth's interior, and this need has led to numerous studies of hydrous minerals under high-pressure and high-temperature conditions. In this issue, Hu et al.¹ (page 241) cast fresh light on the hydrogen-circulation issue. They report that an oxygen-rich iron oxide, FeO₂, is stabilized at pressures greater than about 76 gigapascals, and that this material might enable previously unknown hydrogen and oxygen cycles to occur in Earth's mantle. Click here for the full article