Hydrogen, the most abundant element in the universe, has great potential as an energy source. Unlike petroleum, it can be easily generated from renewable energy sources. It is also nonpolluting, and forms water as a harmless byproduct during use, produces no greenhouse emissions when burned in pure oxygen and is considered by many to be the clean energy of the future and a replacement for fossil fuels. Yet it is so difficult to store that its use as a fuel has been limited. Currently absorption of hydrogen in metal alloys is being researched to absorb up to 1,000 times their own volume of hydrogen but they are heavy and become brittle after repeated use. The hydrogen storage alloys in common use occur in four different forms: AB5 (e.g., LaNi5), AB (e.g., FeTi), A2B (e.g., Mg2Ni) and AB2 (e.g., ZrV2). A gram of hydrogen gas occupies about 11 liters (2.9 gallons) of space at atmospheric pressure, so for convenience the gas must be intensely pressurized to several hundred atmospheres and stored in a pressure vessel. In liquid form, hydrogen can only be stored under cryogenic temperatures. These options are not practical for everyday use. Chinese and American scientists have developed a method of storing high quantities of hydrogen inside tiny tubes of carbon just two nanometres diameter. This is due to the fact that hydrogen is simply adsorbed by the large specific surface of C-nanotubes as it would be the case with any other high specific surface material. The team found that its tubes would absorb one hydrogen atom for every two carbon atoms. What is more, almost 80% of the stored hydrogen could be released from the tubes at room temperature and pressure, with the rest released after the tube was heated. The development is another step in the search for technical solutions to the problems that currently prevent hydrogen from being used as a practical, everyday fuel.