The funicular railway carries me downwards into the mountain, and the familiar, above-ground world vanishes. The tunnel is steep, bored through solid granite, and we gradually descend – down, down, down – for nearly a quarter of an hour. After a while, the pressure makes my ears pop.

When the passageway finally ends, we are 700 metres underground, and I can see a vast subterranean complex of caverns and pipes spread out beneath me. It is quite a sight. But then again, you would expect nothing less from an experiment that could change particle physics forever.

I have come to the Jiangmen Underground Neutrino Observatory, better known as JUNO, a machine in southern China designed to catch ghostly particles called neutrinos. These are the tiniest fragments of reality we know of, so fleeting that they barely exist at all. Studying them has already overturned some of our most basic assumptions about the universe – and there are good reasons to think they hold the key to some of its deepest remaining secrets. “Neutrino mass provides us with the only particle physics evidence that the standard model of particle physics is incomplete,” says physicist Silvia Pascoli at the University of Bologna, Italy.

When it comes to the fundamental ingredients of reality, we know an awful lot about what is out there – but also know that we aren’t yet seeing the whole picture. The standard model of particle physics is a list of these ingredients, the fundamental particles and forces that make up the cosmos. There is an elegant symmetry to it, and it has been nothing short of a spectacular success. But it doesn’t include gravity or explain the nature of dark matter. Then there is antimatter. Every kind of matter particle has an antimatter counterpart, with the same mass but opposite properties, such as electric charge. When matter and antimatter meet, they annihilate each other in a burst of energy. That creates a puzzle because, as far as we understand, the early universe should have produced matter and antimatter in equal amounts – raising the question of why matter now dominates.

Because of these kinds of mysteries, physicists tend to believe that there must be something beyond the standard model – some piece of the puzzle that we are missing. And there is one lead that looks increasingly promising. Neutrinos barely interact with anything – trillions of them pass through your body every second. Yet we know they have a very, very small mass. This mass seems to manifest in strange ways, however, and we aren’t even sure how they get it. If we pull at this thread, might the mysteries of the standard model begin to unravel?

The first hint of neutrinos’ strangeness arose in another underground site: a gold mine in South Dakota. There, in the late 1960s, physicist Raymond Davis Jr. filled a huge tank with a chlorine-containing cleaning fluid to measure neutrinos streaming towards Earth from the sun. Like nearly all neutrino detectors, it was buried to screen out other particles, which would be blocked by the rock, while the neutrinos would pass through. Occasionally, a neutrino would strike a chlorine atom in the tank, transforming it into radioactive argon-37 that Davis could then count. But when he ran the numbers, there was a problem: around two-thirds of the neutrinos physicists had expected never showed up.

Thanks to a gradual series of discoveries, we now know that, like the other matter particles in the standard model, neutrinos come in three types, called flavours. There is the electron neutrino, the muon neutrino and the tau neutrino. “Why nature is made like this, we have no idea, but that’s what we observe,” says Pascoli.

Then, around the year 2000, physicists at other neutrino experiments discovered that neutrinos have an additional peculiarity. As they travel, they can change from one flavour into another, a metamorphosis known as neutrino oscillation. An electron neutrino, the kind produced in the sun, might arrive at Earth as a muon or tau neutrino.

So, Davis’s detector in the gold mine – which had been designed to reveal the effects of electron neutrinos – wasn’t missing two-thirds of its quarry after all. Those neutrinos had simply changed flavour during their journey to Earth. “This was an absolutely gigantic discovery that had been building up for a long time,” says theoretical physicist André de Gouvêa at Northwestern University in Illinois.

The same experiments revealed another weird neutrino property. The oscillation is possible only because the three flavours don’t each correspond to a distinct mass. An electron neutrino isn’t simply lighter or heavier than a muon neutrino; none of the flavours has a single, well-defined mass. Instead, there are three neutrino “mass states”, and each is a strange quantum mixture of all three flavours at once. As they travel through space, the balance between these masses – and therefore the flavour we observe – changes. But what exact values make up that mixture still elude us.

Confused? If so, you’re in good company. “So, in the standard model, neutrinos don’t have mass – but now we know they do,” says physicist Anezka Klustova at Imperial College London. “But we don’t know what those masses are, and we don’t know how they get them.”

The fact that the masses are extraordinarily small adds to the intrigue. The disparity between the mass of an electron and a neutrino is roughly the difference between a fully loaded station wagon and a single playing card. For physicists, this vast gulf raises the question of whether neutrinos acquire their mass in a different way from other matter particles, whose masses arise through their interaction with the Higgs boson.

If so, that could fling open a new window on reality. Some proposed mechanisms for generating neutrino mass would violate rules about how reality functions that physicists currently assume to be true. That, in turn, could shed light on deeper puzzles, including the question of why matter won out over antimatter. “It is possible that whatever is giving the neutrinos a non-zero mass is also directly responsible for the origin of matter itself,” says de Gouvêa.

Order to the masses

These questions about neutrino mass prompted the construction of JUNO. And one thing is for sure: we have come an awfully long way from Davis’s glorified tank of cleaning fluid. JUNO’s centrepiece is a vast acrylic sphere filled with 20,000 tonnes of a special fluid called a scintillator. About 53 kilometres away, the Yangjiang and Taishan nuclear power plants continuously produce a powerful and very clean stream of electron antineutrinos – the antimatter partners to electron neutrinos. A fraction of the antineutrinos are piped to JUNO, and when one interacts with the scintillator fluid, it produces a tiny flash. Thousands of spherical, golden orbs are positioned around the edges of the sphere to amplify these flashes.

The scale of the engineering takes my breath away. The cavern housing the detector could swallow a 14-storey building. Sadly, when I visit, the whole structure has been draped in black plastic – although perhaps it is fitting that this machine searching for the most elusive particles is itself hidden from view.

One of JUNO’s first goals is to resolve a tricky aspect of the puzzle around neutrino masses. Although we don’t know the exact masses of the three neutrino mass states, we do know that two of the states appear to be close together. This leaves open two possible configurations of the mass states: the normal and inverted hierarchies (see graphic below). These mass states are imprinted onto ordinary neutrinos and antineutrinos in the same way, so the fact that JUNO works mainly with the latter isn’t an issue.

Working out which hierarchy is the right one could help constrain what the individual masses actually are and narrow competing explanations of how neutrinos get them. One of the most intriguing possibilities is known as the “seesaw mechanism”, first proposed in the 1980s. It proposes that undiscovered neutrinos exist that are enormously heavy and therefore extremely difficult to observe. Their gargantuan mass would be linked to the tiny masses of the familiar neutrinos we see today – push one end of the seesaw up and the other drops. “This is an explanation which most of the neutrino theories consider plausible, and it also explains why neutrinos are much lighter than the [other matter particles],” says Pascoli.

Another possibility is that neutrinos get their masses indirectly, through interactions with other particles that have yet to be discovered. Their mass would emerge only through a more complicated quantum process, naturally making it much smaller. But intriguingly, in some versions of this idea, one of those undiscovered particles could be the mysterious dark matter that makes up most of the matter in the universe. It is important to say that both hypotheses are broad-brush and don’t offer exact predictions.

Still, JUNO should be able to help us. As the antineutrinos travel the 53 km odyssey from the power plants to the detectors, the different masses in the superposition leave behind a delicate interference pattern in the neutrinos’ energies: a succession of tiny peaks and dips whose precise positions depend on whether the third mass state lies above or below the other two. Measure that pattern accurately enough, and JUNO can begin to tease out the ordering. A smaller companion experiment, the Taishan Antineutrino Observatory, sits much closer to the Taishan reactor. There, it can measure the antineutrinos before they have had much of a chance to oscillate, giving JUNO a reference.

In June, the JUNO Collaboration published results that put it on track to produce a competitive measurement of the mass ordering in the next few years. These initial results lean towards the normal ordering, but more data will be needed before physicists can draw a firm conclusion.

Even then, we probably won’t have a bulletproof answer on how neutrinos get their mass. Instead, JUNO will act like a sieve, making some theories look more plausible while putting others under strain. If the normal ordering result does hold up, for instance, that would fit naturally with some of the simplest seesaw models. An inverted ordering, by contrast, could rule out those versions or force theorists to make them more elaborate. “By doing these neutrino experiments, I don’t think we’d learn all the answers, but we would get a lot of circumstantial evidence that points us in the right direction,” says de Gouvêa.

The neutrino revolution

In a sense, JUNO is just the start of what could be a neutrino revolution in physics. Hot on its heels are two other epic detectors: Hyper-Kamiokande in Japan, which is set to start running next year, and DUNE in the US, due to open in the early 2030s. Both experiments will also be able to probe the neutrino mass ordering. But more importantly, they will search for whether neutrinos and antineutrinos behave differently.

Both Hyper-Kamiokande and DUNE will create beams of muon neutrinos by smashing high-energy protons into a solid target, like graphite. Unlike JUNO, though, the experiments have the luxury of controlling their neutrino source. They can switch their beams on and off and choose whether to produce neutrinos or their antimatter counterparts. The difficulty is that the results they detect will be affected by several tangled factors will affect the results, including the neutrino mass ordering. If JUNO can pin that down first, Hyper-Kamiokande and DUNE will have one fewer unknown to disentangle. With these three detectors working together, we could finally have answers to the neutrino riddles that have plagued us for so long.

The answers may hold sway far beyond the realms of particle physics. Neutrinos’ individual gravitational influence is negligible, but there are so many of them that, over cosmic time, they can affect how matter gathers into clusters, galaxies and stars. That makes them unusually interesting, given that our cosmological models are starting to show signs of strain.

Take dark energy, the unknown force that appears to be accelerating the expansion of the universe at an ever-increasing pace. The accepted understanding has long been that dark energy is a constant. But last year’s results from the Dark Energy Spectroscopic Instrument seemed to show that it may, in fact, be weakening. And there are other tensions, too. Matter in the universe appears, by some measures, to be slightly less clumped than expected. This suggests that something may have suppressed the growth of the universe’s structure over billions of years.

Could that something be neutrinos? Possibly. Because they move rapidly through the early universe, they resist being trapped inside growing clumps of matter and can smooth out structure. The heavier neutrinos are in total, the stronger that effect becomes. All of this means that the results from experiments like JUNO will help those studying the universe check whether their models make sense. “Neutrino properties can be very helpful in tensions like these,” says cosmologist William Giarè at the University of Hawaiʻi.

Going supernova

There could also be one final curtain call for JUNO. The hope is that it will run long enough to catch an extremely rare event: a star exploding somewhere in our galaxy. “The reason we want to have a 30-year lifetime is exactly for the supernova neutrinos,” says Yifang Wang, JUNO’s director. “We want to wait for one explosion.”

When a massive star collapses, neutrinos escape from its core before light can fight its way out. As such, a burst of supernova neutrinos detected on Earth would give us an extraordinary glimpse of the star’s final moments – and a fresh way to test our theories of how supernovae form. Plus, if the collapsing core is massive enough to form a black hole, the torrent of neutrinos could suddenly stop. Physicists would, in effect, watch the signal from the dying star switch off at the moment the black hole forms. “If a supernova goes off, that would be the physics result of the century,” says Pascoli.

Eventually, it is time for me to leave JUNO. I am led through a baffling series of tunnels to a cramped lift, where my guide pulls a chain-link curtain across the entrance. With a jolt, we shoot upwards, the lift screaming past an endless wall of rock. A few minutes later, I step back into daylight, almost 2 km from where I first stepped onto the funicular. It is a final reminder of JUNO’s improbable scale – complete with a detector weighing as much as three Eiffel Towers – all to study nature’s tiniest particles. Perhaps, in the next decade, a seismic shift in physics will come from its shyest, most diminutive character.