The view into the 6.3-kilometre-long HERA tunnel shows the superconducting magnets for guiding the protons on top and the electron ring beneath them.
<a href="//commons.wikimedia.org/wiki/User:DESY-Kommunikation" title="User:DESY-Kommunikation">DESY-Kommunikation</a>
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<span class="int-own-work" lang="en">Own work</span>