Abstract From the very beginning, nature used hydrothermal processes to create fuels during various geological and biological eras involved in the formation of our planet. We then intensively used such fuels for societal development. Like any inheritance, we have the responsibility to pass them on to further generations. However, the rate at which we have used such nonrenewable fuels is enormous, despite their initial abundance. As a consequence, Earth’s environment today is in danger because of dramatic increase in CO2 associated with burning fossil fuels. Although, as few as 300 years ago humans subsisted almost entirely on renewables, our habits have progressively changed, and by the end of the twentieth century we became 100% dependent on fossil fuels. Besides damaging the ecosystem, such fossil resources are unevenly distributed geographically, and therefore, the price of oil today bears no relationship to its production cost. We clearly need to move away from fossil fuels in order to prevent further damage of the planet. A viable and environmentally safe alternative to fossil fuels needs to be available for the future generations. As nature made fossil fuels hydrothermally, it makes sense to learn from such natural processes. Biomass is the most available renewable resource on Earth, and therefore it should become the main source for future renewable energy. In this chapter, we present an overview of sustainable and viable hydrothermal transformations of biomass into artificial coal and related materials, or hydrothermal carbon.