Even the Large Hadron Collider (LHC) needs a break from time to time.

Scientists with the European Organization for Nuclear Research (CERN) flipped the switch on the LHC this week, the beginning step in a two-year effort to revamp the particle collider to accommodate intense future experiments that will require higher energy levels than previously used. At the end of that two-year period, in early 2021, the plan is to switch the particle collider back on again.

Once back online, the LHC will be outfitted with two new accelerators, which will pack enough punch to shoot two particles together harder than ever before, hopefully unlocking some of the mysteries of physics.

Built between 1998 and 2008, the LHC is a 17-mile-long (27 km), circular tunnel that sits close to 600 feet (175 meters) beneath the French-Swiss border near Geneva. The collider was created to be a tool for physicists to test their theories about particle physics. Its last two-year maintenance period took place between 2013 and 2015.

The collider has already advanced physics in significant ways. In 2012, scientists used it to observe for the first time a particle known as the Higgs boson. Physicists in the past had predicted its existence, and believe it to be essential for atoms to form. The work wound up winning a Nobel Prize in physics in 2013. Presently, scientists have been using the LHC to analyze the way the Higgs boson particle decays or transforms into other particles.

Just because the collider is temporarily shuttered doesn’t mean the scientists that work with the machinery will be taking a vacation. According to a statement by CERN, researchers “will be busy throughout the shutdown examining the huge data sample for possible signatures of new physics that haven’t had the chance to emerge.”

While this shutdown is related to planned upgrades and maintenance, the LHC has been forced out of service several days in the past, most recently because of snafus with local wildlife. In April 2016, a weasel climbed into the machinery and caused what was described to the BBC as a “severe electrical perturbation” when it got into a 66 kilovolt transformer. Before that, in 2009, the LHC was temporarily closed down after a bird got into the equipment.