The European X-ray Free-Electron Laser (XFEL), the largest x-ray laser in the world, has generated its first laser light, reaching the last major milestone before its official opening in September. A view into the 2.1-km- long accelerator tunnel of European XFEL with the yellow superconducting accelerator modules hanging from the ceiling. Courtesy of DESY/D. Nölle. The 3.4-km-long facility is located mostly in underground tunnels. The x-ray light has a wavelength of 0.8 nm, about 500× shorter than that of visible light. At first lasing, the laser had a repetition rate of 1 pulse/s, which will later increase to 27,000/s. "This is an important moment that our partners and we have worked toward for many years,” said Robert Feidenhans’l, managing director of the European XFEL. “The European XFEL has generated its first x-ray laser light. The facility, to which many countries around the world contributed know-how and components, has passed its first big test with flying colors. The colleagues involved at European XFEL, DESY and our international partners have accomplished outstanding work. This is also a great success for scientific collaboration in Europe and across the world. We can now begin to direct the x-ray flashes with special mirrors through the last tunnel section into the experiment hall and then, step by step, start the commissioning of the experiment stations. I very much look forward to the start of international user operation, which is planned for September." This is the first laser light at the European XFEL, recorded by an x-ray detector at the end of the tunnel. Courtesy of DESY. The x-ray laser light of the European XFEL is extremely intense and one billion times brighter than that of conventional synchrotron light sources. The achievable laser light wavelength corresponds to the size of an atom, meaning that the x-rays can be used to make pictures and films of the nanocosmos at atomic resolution, such as of biomolecules, from which better understandings of the basis of illnesses or the development of new therapies could be developed. Other opportunities include research into chemical processes and catalytic techniques, with the goal of improving their efficiency or making them more environmentally friendly; materials research; or the investigation of conditions similar to the interior of planets. The x-ray laser light of the European XFEL was generated from an electron beam from a superconducting linear accelerator, the key component of the x-ray laser. The German research center DESY, the largest shareholder of the European XFEL, put the accelerator into operation at the end of April.