Straddling the French-Swiss border, almost 600 feet below the ground, is a 16-mile-around, circular tunnel.

Resting in that tunnel is the world’s biggest atom smasher: The Large Hadron Collider (LHC). This year, marks the 10-year anniversary of the LHC’s most incredible discovery, a discovery that shook the world and solidified a theory of the universe that had been hanging in the balance for 50 years. On July 4, 2012, the LHC found the Higgs boson.

In this month’s Century of Science theme, Cracking the Atom, Science News looks back on how a smattering of particles turned the world of physics – and ultimately the universe – upside down. In the 1920s, physicists thought the subatomic world was beautiful simplicity, just a proton and an electron as the smallest units that made the entire universe possible. It was so simple and elegant that most mainstream physicists refused to even consider the possibility that other particles might exist.

Yet, there were curiosities and uncertainties about the nature of mass, like a pesky mosquito in your ear. These questions kept some physicists wondering whether there just might be another particle or two. By 1932, physicists found two new particles – the neutron and the positron.

Then all hell broke loose.

Over the next 30 years, new particles were discovered by the dozens. The proton and the electron were no longer the smallest particles in existence. Rather, they were made of even smaller particles that scientists could observe when they accelerated protons or electrons to fantastic speeds and then smashed them together (early versions of the LHC), leaving behind a wreckage of subatomic particles.

Not only were physicists dealing with the onslaught of new subatomic particles, they were also dealing with some very weird phenomena. See, the subatomic world isn’t like our world; it’s a weird world. In our world, separating two things removes their connection to each other. In the quantum world, two particles are always connected, regardless of how far apart they are. Physicists devised explanations for the mind-bending phenomena they saw, defining things that must exist but had not yet been found.

One such theorized entity is the Higgs boson. In the 1960s, physicists were struggling to understand how subatomic particles gained mass. Mass is what makes particles “stick” to each other. Without mass, all particles would fly around the universe at the speed of light and would never “stick” together to create things like stars, planets, horses, people, cell phones, etc …

Peter Higgs (the boson’s namesake), devised an elegant – but theoretical – solution. Imagine that the entire universe is covered in a mesh. As particles move through the universe, they can either fly directly through the holes in the mesh or they can hit the webbing of the mesh. Each time they hit the mesh, the particle slows down. This mesh is called a Higgs field, and the more a particle hits the mesh, the more mass the Higgs field endows into the particle.

The intriguing idea that Higgs had, however, was that physicists could prove this mass-endowing field existed by observing a byproduct of the Higgs field: the Higgs boson.

So, what is the Higgs boson? The Higgs boson is a subatomic particle that emerges from the Higgs field. The Higgs field is buzzing with energy, and sometimes that energy concentrates in one spot. This concentration of energy, like bees swarming to a soda can, is the Higgs boson.

Wait. Isn’t the Higgs boson a particle? How is it both energy and a particle?

Because all particles are just concentrations of energy in a field, and all fields have particles.

See, the Higgs field isn’t the only field that exists. All particles – electrons, protons, neutrons, quarks and more – emerge from concentrations of energy that occur in their own fields. The universe is actually built on layers and layers and layers of these fields that are constantly flowing, concentrating and exchanging energy. As energy concentrates in one area of a field, a particle appears. This is called quantum field theory.

Peter Higgs’ theory of the Higgs field, which necessarily implied the existence of a particle (the Higgs boson), was a luminary extension to quantum field theory that explained how subatomic particles gained mass.

Fast forward to 2012 – physicists have spent the last 50 years searching for the Higgs boson. While the atom smashers that existed previously were impressive, none could accelerate protons to the speed necessary to break down into a Higgs boson, but the Large Hadron Collider could.

By accelerating protons to nearly the speed of light, whipping them around and around the 16.7-mile racetrack, physicists smashed the protons together and released a firework-like display of subatomic particles. Within that display was a noticeable anomaly: a fantastically large particle that quickly broke down into different subatomic particles, exactly what was expected of a Higgs boson.

After numerous repeats and analyses, the scientists proved to themselves that the anomaly was in fact a Higgs boson. They released their results in 2012 to a world astonished.