“Don’t lose sight of me.”

Physicist Abhigyan Dasgupta — “Riju” to his family and friends — looks over his shoulder to caution our 12-member group as we prepare to descend 300 feet beneath the French countryside into the largest machine on Earth: the $4.7 billion Large Hadron Collider.

Big Science doesn’t get much bigger than the Large Hadron Collider (LHC) on the campus of the European Organization for Nuclear Research (CERN) facility outside Geneva. The LHC runs in a circular 16.7-mile underground tunnel that crisscrosses the Swiss and French border. More than 1,200 superconducting magnets, each 50 feet long and weighing 35 tons and joined end-to-end, make up the collider’s ring. Guided by the magnets, trillions of protons circulate at nearly light speed in opposite directions inside a pair of two-inch-diameter tubes. At four points along the ring, aiming magnets send the two counter-rotating beams crashing into each other.

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These collisions blast the protons into subatomic smithereens. Detectors weighing thousands of tons — think of them as giant cameras — positioned at the collision points record three-dimensional images of each of the 40 million proton mash-ups that occur every second. Aided by high-speed computers and algorithms of mind-bending complexity, scientists pore over the tracks left by these impacts for clues about the origins of our universe and the existence of extra dimensions, and for undiscovered particles that might usher in a new era of discovery in the physical sciences.

Our gateway to the collider is the Compact Muon Solenoid (CMS) on the accelerator ring near Cessy, France, a half-hour car ride from downtown Geneva. The CMS detector is one of seven experiments on the collider’s ring and one of the three that visitors may tour.

We enter a cavernous hangarlike room dominated by a 70-foot-high cross-sectional photograph of the CMS detector deep beneath our feet. When the accelerator is running, we are told, “this is as close as visitors would get to the CMS.”

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The Large Hadron Collider is in the first months of a two-year technical shutdown for maintenance and upgrades. During that time, visitors lucky enough to secure a place on a CERN guided tour can go deep underground to visit the cavern that houses the gigantic detectors. When researchers switch the collider back on in early 2021, visitors will receive a more limited underground tour until the next two-year stop in 2025.

“You’re extremely lucky,” Dasgupta tells us. “If you had come here a year ago, you would not have been able to go down.”

We round a corner and stop in front of a blocked-off entrance. Dasgupta steps into something that looks like a yellow phone booth. He looks intently at a retinal scanner on the wall of the booth. The doors close behind him as another set opens on the other side. “It scans my eyes and it lets me in. The rest of you get to be cargo and go through the cargo door.”

Dasgupta lets us in and we board the freight elevator that will take us 300 feet down to the detector chamber. At the bottom, we pick up orange hard hats fitted with a computer chip so that security can track our precise location. We walk along a long passageway, its walls hung with photos of the collider and informational posters.

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Dasgupta, a natural teacher who has just completed his physics PhD at UCLA, switches into data-delivery mode. He explains that, at full power, a proton makes 11,245 trips around the collider ring every second. Instead of a continuous stream, the protons travel in bunches, or packets. Each packet contains 110 billion protons and is spaced 25 nanoseconds apart, an interval of about 25 billionths of a second. Before a proton is annihilated, it will be traveling at 99.9999991 percent the speed of light, he says.

The next 15 minutes pass as a blur. We must hurry; there are other tours behind us. We pass banks of electronic equipment and computers that tower over our heads, all connected by rivers of wires that snake up to the ceiling.

It is here, Dasgupta says, where electronic hardware and computer software winnow the 40 million collisions that occur every second to the 1,000 most interesting ones. These are stored for further analysis.

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We scurry through narrow passageways — banks of overhead lights illuminate our way past danger warning signs and radiation symbols — and descend steep stairs. One more suspended catwalk takes us into the underground cavern that houses the CMS.

The detector is split horizontally in half like a giant melon. The two halves are pulled apart to allow workers access to the detector’s inner workings. They will be inched back together and the proton beam tubes reunited when the work is complete.

It’s anything but compact. The detector measures nearly 70 feet long and 50 feet wide and, at 14,000 tons, weighs about twice as much as the Eiffel Tower.

It’s also beautiful. Concentric rings of gold and candy-apple red detectors encircle the proton beam tubes at its center. Lime-green supports help hold the electronics in place. Shimmering metal that looks like aluminum foil catches the industrial lights. It could be a space station. Or a half-billion-dollar piece of art.

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We can almost touch it. It’s so close and so big and so everywhere that it’s hard not to be awed, overwhelmed and a bit intimidated. I can do little more than stare.

“We need to go,” Dasgupta says, and we begin our journey to the surface.

CERN each year draws researchers from 23 countries and several hundred thousand tourists to its campuslike grounds at the foot of the Jura mountains. The Dalai Lama has paid a visit. So have Metallica and King Juan Carlos of Spain. Tom Hanks, Al Gore, Richard Branson, Patti Smith and the Black Eyed Peas also have toured the collider.

The acronym CERN is derived from the French words Conseil Européen pour la Recherche Nucléaire. CERN was founded in 1954 as one of Europe’s first joint ventures after World War II. Right now, 23 member countries together pay $1 billion a year to keep CERN’s lights on and the molecules spinning.

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Work on the CMS experiment began in 1999. Dasgupta said the detector recorded its first proton collisions in 2009. Three years later, CERN rocked the scientific world when its researchers announced that CMS and ATLAS, its sister experiment on the collider’s ring, had discovered the elusive Higgs boson. The discovery ended a 50-year search for what became known as the “God Particle,” the last piece needed to validate the Standard Model of particle physics, the theory that explains three of the universe’s four fundamental forces — except for gravity — and all its elementary particles.

The announcement was made in a packed CERN auditorium on July 4, 2012 — a date that some physicists celebrated as “Higgsmas.” Today, an empty bottle of champagne, opened and drained by those researchers, is displayed on the wall of the Microcosm, the museum-like permanent exhibit in the CERN reception center that takes visitors on a virtual tour of the collider and its key experiments.

“We get tour requests for three times more visitors than we can accept,” said Francois Briard, CERN’s head of visitors and local engagement.

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Briard and his staff of four apportion out available spots on guided tours. School groups get top priority. Next they try to accommodate requests for groups of 12 or more. Individuals get what’s left. Last year, school groups filled about two-thirds of all tours while the remaining slots fell about equally to non-student groups and to individuals. The tours are led by volunteer guides, nearly all of whom are CERN researchers.

“We cannot guarantee you will go underground. But there are wonderful things to see on the surface,” Briard said. “We have 20 places to show visitors; only five are underground, and these five are inaccessible much of the time.”

Today, even visitors who don’t know a proton from a futon can get caught up in the sense of discovery and wonder as they stroll past billions of dollars of exotic technology and peer over the shoulders of working scientists as they attempt to unlock nature’s most closely guarded secrets.

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“This is a place where science takes place,” Briard said. And the view of science from the inside can be breathtaking.

To visit CERN, he said, is to “be amazed.”

Morin is a writer based in the District. Follow him on Twitter: @richmorin.

More from Travel:

If you go

Where to stay

N.H. Geneva Airport Hotel

Av. De Mategnin 2, Geneva

011-41-22-989-90-00

Many people who visit CERN stay in Geneva. But if you want to stay closer, this hotel is a mile and a half from the CERN reception building and the Globe. It features a restaurant, bar and fitness center with sauna. Rooms start at about $125 per night.

Hotel Cornavin

Blvd. James-Fazy 23, Geneva

011-41-22-716-12-12

This four-star hotel is across the street from the Gare Cornavin train station. It’s just a 20-minute train ride from the station to the CERN campus. The hotel features 164 well-appointed, air-conditioned rooms (air conditioning is not always a given in Swiss hotels). Rooms start about $125 per night.

Hotel Kipling

Rue de la Navigation 27, Geneva

011-41-22-544-40-40

This modern and impeccably maintained hotel in the heart of old town Geneva is a five-minute walk to the Cornavin train station. Walk a few minutes longer and you’re at Lake Geneva. Rooms start at about $135 per night.

Where to eat

Les 5 Portes

Rue de Zurich 8, Geneva

011-41-22-731-84-38

This unpretentious restaurant located a short walk from both the train station and Lake Geneva offers a delicious mash-up of Swiss, Japanese and Italian-inspired dishes. Scallops with miso is the signature entree, but don’t miss the stuffed artichoke with baby vegetables appetizer. Entrees start at about $22.

Le Cafe de la Gare

Rue de Montbrillant 2, Geneva

011-41-22-733-77-84

This comfortable restaurant directly across from the train station features typical brasserie staples such as beef bourguignon, calf’s kidneys and blood sausage. Cheese-aholics can sample a traditional Swiss cheese fondue or raclette. Fondue/raclette about $26 and other entrees start about $22.

What to do

European Organization for Nuclear Research (CERN)

Espl. des Particules 1, Meyrin

Didn’t win a golden ticket for a guided tour? The CERN campus has much to see and do — and admission is free. Thoughtfully designed exhibits, videos and hands-on displays inside the Microcosm in the reception building tell the story of the Large Hadron Collider and its experiments. Six exhibition areas in the Globe of Science and Innovation explain CERN’s mission, while every half-hour a multimedia “immersive audiovisual experience” tells the story of the universe in six minutes. Bikers and hikers can circumnavigate the full 16.7 miles of the Large Hadron Collider on a route above the LHC ring. Ten exhibition platforms above key points on the LHC explain what’s going on below. Tickets for individual tours go fast; half of the tickets are made available online 15 days before the tour date, and half are released days before. Tours are first come, first served beginning 8:30 a.m. Swiss time (2:30 a.m. Eastern time). Non-school groups of 12 or more can request tours nine months in advance, school groups three months. Two- to three-hour tours offered Monday through Saturday; free.

Information

R.M.