Hadron–Electron Ring Accelerator

Particle accelerator built under the streets of Hamburg, Germany, occupying a tunnel 6.3 km/3.9 mi in length. It is the world's only collider of protons and electrons, designed to accelerate protons to energies of 820 GeV (billion electron volts), and electrons to 30 GeV. HERA began operating in 1992.

HERA can propel electrons into the proton interior, where they interact with one of the proton's three constituent quarks.