When a 2,000-year-old Roman shipwreck was found off the Sardinian coast in 1988, it didn’t just thrill archaeologists — physicists were excited too.
The discovery grabbed the attention of one in particular: Ettore Fiorini, a particle physicist with Italy’s Institute for Nuclear Physics (INFN).
He didn’t care too much about the ship. He was more interested in its cargo — hundreds of lead bars, each weighing 33 kilograms.
And instead of displaying them in a museum, he planned to melt them down to build an underground observatory.
Italy’s Institute for Nuclear Physics agreed to help fund the recovery expedition. (Supplied: CUORE Collaboration and LNGS/INFN)
Dr Fiorini reached out to cultural heritage officials with a proposal. The INFN would help fund the recovery of the ingots if they could keep some for themselves.
He believed this ancient lead could play a vital role in uncovering the very nature of the Universe.
Why do physicists want ancient lead?
Ancient lead is useful for sensitive physics experiments because it has lost the radioactivity that can complicate observations.
When trying to observe elementary particles, which are the tiniest building blocks that make up reality, physicists need to silence any background noise.
The Cryogenic Underground Observatory for Rare Events (CUORE) observes neutrinos and can also search for dark matter. (Supplied: CUORE Collaboration and LNGS/INFN)
For example, particle detectors are often kept deep in caves underground to avoid showers of “cosmic rays”.
These are high-energy particles that come from space, and while they aren’t harmful to humans, they can disrupt experiments.
“Every second of our life, every centimetre of our body is crossed by a particle,” Paolo Gorla, an INFN physicist, says.
“[Going underground] gives us some kind of cosmic silence.”
Laboratori Nazionali del Gran Sasso (LNGS) is the largest underground research centre in the world. (Wikimedia Commons: TQB1/CC BY-SA 4.0)
The detector also must be shielded from the radioactive “noise” that comes from within the cave itself.
“The presence of a human or just the rock of the mountain, or even the banana I bring to eat on a break, can disturb the experiment,” Dr Gorla says.
Lead is a suitable shield from this radioactivity — which can come from cosmic rays or bananas — because it’s super dense.
But freshly mined lead has some radioactive “noise” of its own, because it naturally contains a trace amount of the unstable isotope lead-210, which releases energy as it decays.
“So I [can] build a lead shield to stop the particles [coming from the cavern], but the shield itself generates other particles that disturb the experiment,” Dr Gorla says.
The radioactivity in the lead will fully decay, producing stable lead that’s perfect for use as a shield — but only after a few hundred years.
Loading…
Which is why, according to metallurgist Kevin Laws of the University of New South Wales, physicists are on the lookout for lead mined during ancient Roman times.
It has had plenty of time to become stable.
“But there is debate that by utilising lead from sources such as shipwrecks we are destroying historical items and record,” Dr Laws says.
A battle between the past and the future
In 2012, after underwater cultural heritage researcher Elena Perez-Alvaro gave a talk at a conference, a physicist pulled her aside.
“She said, ‘We are using metal that we have found underwater, some ancient lead, to do our experiments,'” Dr Perez-Alvaro says.
While in some cases this lead was collected ethically, in others it was bought illegally from private companies that didn’t follow archaeological standards.
Loading…
“Everything that is taken out of the water without a proper archaeological record, we will never have that information back,” she says.
“Where the ship was coming from, where was it going. This is basic information to understand the past.”
Dr Perez-Alvaro sparked a heated debate when she wrote papers discussing the dilemma of using historical artefacts for physics experiments.
“It was like a war [between] the ones that defended the past and the ones that defended the future,” she says.
But even Dr Perez-Alvaro concedes there is a case to be made for using ancient lead in experiments — after it has been properly documented and recovered.
“We have to consider that sometimes it’s not useful to have 1,000 ingots in the warehouse of a museum.”
Searching for dark matter
The reason Dr Fiorini was so keen to get his hands on that sunken Roman lead was to shield his experiment as it looked for hard-to-detect phenomena — the most elusive of which is dark matter.
Dark matter is thought to make up 85 per cent of the total mass of our Universe. It’s an invisible substance that doesn’t interact with light but does interact with gravity.
So while we can’t see it, we do have indirect evidence that dark matter exists. In the 1930s, astronomers started noticing gravitational anomalies that suggested there was more mass in the Universe than we could see.
The Swiss-American astronomer Fritz Zwicky called it “dunkle materie”, or dark matter.
Spiral galaxy UGC 2885 is named after Vera Rubin, the American astronomer who made important dark matter observations. (NASA/ESA/B)
It was American astronomer Vera Rubin who brought dark matter into the mainstream in the 1970s with her observations of spiral galaxies.
Since then, physicists have been looking for ways to detect dark matter directly. Until they do, it remains hypothetical.
“We’re building experiments on Earth looking for dark matter to interact with ordinary matter,” astroparticle physicist Theresa Fruth of the University of Sydney says.
Dr Fruth has been working on the LUX-ZEPLIN experiment in the US.
Dr Theresa Fruth preparing to go down into the LUX-ZEPLIN experiment in South Dakota. (Supplied: University of Sydney)
The chamber of the experiment is full of liquid xenon. If a dark matter particle bumps into a xenon atom, like billiard balls colliding, then the detector will pick up a tiny flash of light.
We don’t know if dark matter is capable of interacting with xenon. If it does, it’s a rare occurrence.
“We’re going to run a detector, which is seven tonnes [of xenon], for 1,000 days and we expect maybe a handful of events,” Dr Fruth says.
For those interactions to be observed, the detector must be well shielded from outside radiation.
Researchers in Italy are hoping to make a similar observation at the Cryogenic Underground Observatory for Rare Events (CUORE) under the Gran Sasso mountain.
The CUORE experiment is so sensitive the presence of humans can disrupt its observations. (Supplied: CUORE Collaboration and LNGS/INFN)
CUORE, which is Italian for “heart”, is the experiment protected by a shield of ancient Roman lead.
The ingots were collected with the permission of cultural heritage officials, and documented before being handed over to the INFN in 2010.
Two thousand years ago, this lead would have been used to construct aqueducts or ammunition for soldiers.
The historical importance isn’t lost on Dr Gorla.
“Something amazing is that the companies that extracted the lead from the mine stamped [their] brand on top of the bricks.”
The bricks were branded in Latin, and this part was preserved for archaeological purposes. (Supplied: CUORE Collaboration and LNGS/INFN)
This part of the brick is chopped off before the remaining lead is melted and moulded into a shield.
While the shield was being constructed, the physicists also measured remaining trace contaminants in the lead, uncovering important historical information.
“It was like an ID card … it helped the cultural heritage officials reconstruct from which mine in Spain this lead was extracted,” Dr Gorla says.
He calls it a “mutual exchange” between the archaeologists who wanted to know more about the history of Ancient Rome, and the physicists who want to know more about the history of the Universe.
The CUORE experiment runs at temperatures close to absolute zero, which is the lowest possible temperature. (Supplied: CUORE Collaboration and LNGS/INFN)
The CUORE detector is kept inside dilution refrigerators that keep the experiment “colder than outer space”, according to Dr Gorla.
“At this temperature, a particle passing through one of our detectors can rise the temperature enough to be able to measure it,” he says.
“The way we have to look at particles is different to the way we look at things with our eyes.”
The point of all of this is to detect and observe the elementary particles responsible for giving our Universe structure.
Scientists posing with part of the ancient lead shield for CUORE. (Supplied: CUORE Collaboration and LNGS/INFN)
CUORE started observing in 2017, and six years later, Dr Fiorini died.
The experiment he shielded with Roman lead hasn’t made any major discoveries yet, but it will soon be upgraded to CUPID — which stands for CUORE Upgrade with Particle Identification. The shield will remain in place.
“We can easily tell that without the quality of the shield, we would not have been able to measure at the level we’re measuring now,” Dr Gorla says.
Australia could confirm controversial finding
There is another observatory under the same Gran Sasso mountain in Italy that has been the centre of controversy for decades.
Not because it uses metal from shipwrecks, but because for 20 years it has been detecting a signal the team believes could be dark matter.
Other teams have tried to replicate the experiment, called DAMA/LIBRA, with different techniques and haven’t been successful.
“To once and for all say, ‘Are they actually seeing something?’ We need to build a detector which is really similar,” Dr Fruth says.
That’s the aim of the SABRE South detector in Victoria’s Stawell gold mine, which is on track to start observations in early 2026, and hopefully confirm if the DAMA/LIBRA signal is a true candidate for dark matter — or an error.
SABRE South will run one kilometre underground in the Stawell Gold Mine. (Supplied: ARC Centre of Excellence for Dark Matter Particle Physics)
“It’s just this really big problem … We don’t know what 85 per cent of the matter in our Universe is made out of,” Dr Fruth says.
“Understanding that will help us understand the world a little bit better, and I like to think maybe we’ll understand our place within it a little bit better as well.”
Listen to ‘A battle between the past and the future’ on The Science Show. Tune into Radio National at 12pm on Saturdays to hear the full program or subscribe to podcast for more mind bending science.