[…] building a huge tunnel at the CERN facility that will collide particles to enable a clean study of the Higgs boson. But Europe is leaving the option of using the tunnel for a future collider that could reach energies nearly 10 times higher than the LHC.
The tunnel would be truly enormous, with a circumference of roughly 100km, meaning a diameter of roughly 30km. That would mean it would pass under the nearby Lake Geneva, requiring much deeper tunnels than those used for the LHC
For comparison, the current LHC has a circumference of 27km