Imagine an ancient particle zooming through space after the explosion of a star.

This is a neutrino.

It's been hurtling through space for billions of years, emanating from a massive dying star — a supernova — eight times bigger than our Sun.

We wouldn't exist without supernovas — they contain the building blocks of life. So by learning about neutrinos, we can explore our own origins.

But these ghost-like particles are smaller than an atom, and because they barely interact with anything, spotting them is virtually impossible.

Unless you've got one of these...

Super Kamiokande

Underneath a mountain in central Japan hides this $140 million experiment.

We're inside a tank which is normally filled with water, so there wouldn't usually be people around. But we've been given access whilst it's down for maintenance.

Lining the walls are thousands upon thousands of gold-hued detectors.

These large bulbs are the key to observing an elusive neutrino. They're lightbulbs in reverse — instead of making light, they capture it.

In a regular lightbulb, electricity goes in and light comes out. With one of these detectors, light goes in and electricity comes out.

The tank is fitted with 11,000 of these bulbs, each larger than a human head.

They're so sensitive that if they were on the moon, they could see a match being lit from Earth.

As neutrinos fly through the tank from their origins far away...

...scientists wait patiently outside for one to slam into a molecule of water and set off the detectors.

Suddenly, they spot a tiny burst of light — a neutrino and a water molecule collide, creating a minuscule flash invisible to human eyes.

Scientists have only ever detected 24 supernova neutrinos.

From a single supernova.

32 years ago.

Now, with a major upgrade, scientists are hoping to capture five every year by looking 35,000 times further into space.

What used to be one of Japan's biggest zinc mines is now home to one of its most successful science experiments.

As we make the 2-kilometre journey deep inside the mine near the town of Kamioka, I'm reminded that two Nobel prizes have come from the observations inside this mountain.

My tour guide is experimental physicist Mark Vagins, a charismatic and friendly professor renowned for his colourful shirts.

"Welcome to my secret underground lair," he tells me as we get to his lab.

"The real trick is keeping the lasers on the sharks," he jokes.

Inside the Kamioka Observatory, there are two giant caverns.

One used to house Super Kamiokande's antecedent — Kamiokande — which was built in the 80s but was decommissioned in 1997 to make way for a different experiment, which now occupies the cavern.

Super Kamiokande opened in another cavern in 1996 and is much bigger than its now-gone relative was.

When you're inside the tank it's mesmerising — if not a little disorientating.

You could fit the entire Statue of Liberty inside.

The tanks are water Cherenkov detectors — among the "simplest detectors imaginable", according to Professor Vagins.

"They are simply tanks of very pure or very clean water surrounded by light collectors," he says.

The reality is this is anything but simple — we're deep underground and searching for clues to the origins of our universe.

"So when a neutrino comes into the detector, usually it goes through and nothing happens. But every so often in the case of Super K — about once an hour — an extraordinarily unlucky neutrino will plough directly into a water molecule and it will disrupt that water molecule, and when that happens, you get that flash of light."

Not all neutrinos are the same — they come from a variety of places.

Many billions a second are passing through you right now.

Our own Sun makes neutrinos. So do nuclear reactors.

When a supernova neutrino disrupts a water molecule in the tank, it reacts in a manner distinct from neutrinos emanating from other sources, allowing scientists to distinguish it.

And it's these supernovas that are of real interest to Professor Vagins.

"A supernova is a neutrino bomb," he says.

"We would not be alive without supernovas — most certainly supernovas are the best source of neutrinos."

From star to supernova

A star generates heat and light by fusing helium and hydrogen in its core. This is how it functions for most of its life.

But when the star runs out of hydrogen, it begins to die out and expands into a red supergiant star.

If the star is large enough, the layers in the core continue to be consumed by fusing into heavier and heavier elements. Finally, an iron core is formed.

The iron core cannot be further fused so it stops generating outward force to counteract gravity. The core implodes and sends out trillions of neutrinos at near-light speed.

The star then detonates, blasting its elemental riches throughout the universe.

"Every atom of oxygen — every single one that we breathe, that's in the water that we drink, in the water of our bodies — every atom of oxygen was made in the heart of a dead star, an exploded star that's now long gone," Professor Vagins says.

So catching one of these tiny particles is like a window into the past. But it's easier said than done.

Right now, the Super Kamiokande can only see supernovas that occur in and around our galaxy.

Scientists expect they only happen in our area every 30 to 50 years.

It's a patient waiting game for that elusive eureka moment.

And that's where Professor Vagins's research comes in.

He's spearheading the upgrade of the Super Kamiokande to let it see 35,000 times further into space.

And the key to that is the water.

Super Kamiokande has some pretty special water — 50,000 tonnes of the purest stuff on Earth.

In fact, the water is so lacking in impurities, it's corrosive.

Given long enough, it can dissolve metal — something scientists discovered when they dropped a hammer in the tank.

Years later, they found the hammer. The interior was hollow and all that was left was a chrome shell.

"The water had eaten all the metal out of the hammer and left the chrome there," Professor Vagins says.

So this H 2 0 is pretty special, but Professor Vagins believes a much more obscure element holds the secret to unlocking the experiment's full potential.

He and fellow professor John Beacom theorised that by adding a silvery-white rare earth metal called gadolinium to the water, it could make the experiment much more sensitive.

"It's a very strange element; it's very obscure and nobody's really heard of it but it's got very unusual properties," he says.

"The property that we care about is that it loves to eat neutrons more than anything in this world, and when it eats a neutron, when it absorbs a neutron, it makes its own flash of light."

But proving gadolinium would do the trick was quite a feat.

Professor Vagins had to build his own detector — 1/250th the size of Super Kamiokande — to show that it wouldn't destroy the experiment.

But getting around with the stuff hasn't always been easy — as the professor learnt when trying to board a flight in the United States.

"I had a kilogram of this in my carry-on and as it went through the scanner I was watching the face of the woman as it went through … and she made a funny face because apparently, this shows up as a great big black blob as it's a heavy metal and it's opaque to X-rays," he recalls.

"The next thing I knew there were all these men with automatic weapons pointed at me.

"So they weren't happy to find a kilogram of white powder in my luggage.

"I might add that if this ever happens to you, which it could, don't say the following to the arresting officer: 'Don't open that, it's really pure.' So that got me into some trouble."

Border checkpoint hurdles aside, Professor Vagins eventually got his gadolinium to Japan — where it's now going to go inside the Super Kamiokande.

When a neutrino hits this pure water with gadolinium inside, it creates a "gadolinium heartbeat".

"So in one point in space separated by about 30 millionths of a second you get a 'boom boom, flash flash, flash flash' and that double flash is unique and distinctive and allows you to wipe out the background so all of a sudden you have this clear view of a distant sky.

"This will allow us to see supernova neutrinos from a billion years ago."

Since we visited, the tank has now been repaired, re-waterproofed and some broken light detectors were replaced.

Scientists hope the Super Kamiokande with gadolinium will be up and running by the end of this year or early next year.

And Professor Vagins said it would be a win-win situation even if the experiment didn't find any supernova neutrinos.

"Really you can't lose if you measure something everybody wants to measure and learn from — or if you don't see it then you have a whole new world opening up of discovery," he said.

"Neutrinos are the lightest things we know about — so if these things decay that implies new physics, new particles: a whole new underlying world."

A new world and perhaps a third Nobel prize.

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