In the H1 and ZEUS experiments, electrons and quark-filled protons were brought to collision. The electron acts as a pointlike probe that scans the proton. With a resolution of 10-18 meters, corresponding to a thousandth of the proton diameter, HERA is the best electron microscope in the world. With these electron-proton collisions, it is possible to measure the structure of the proton and the strong force acting within it very precisely. One special feature of HERA collisions is the fact that the electrons do not influence the measurements because they react to other forces than the quarks in the proton. The HERA measurements for the first time confirmed the nature of the strong force, as it was predicted 20 years ago by the physicists Davis Gross, David Politzer and Frank Wilczek. For this discovery they were awarded with the Nobel Prize in 2004.
It is a bit sad to see many of these wonderful and historically significant facilities go. This is not uncommon, because many facilities throughout history became obsolete, being replaced by bigger, better, and more powerful facilities.