Why the Search for Dark Matter Depends on Ancient Shipwrecks

Errant particles from everyday radioactive materials are a major obstacle for particle physicists. The solution? Lead from the bottom of the sea.

The wreck of the S.S. Thistlegorm, which sank on October 5, 1941, in the Red Sea
Cigdem Sean Cooper / Getty

In 2017, Chamkaur Ghag, a physicist at University College London, got an email from a colleague in Spain with a tempting offer. The year before, an emeritus professor at Princeton  University, Frank Calaprice, had learned of old Spanish ships that had sunk off the New Jersey coast 400 or 500 years ago, while carrying a cargo of lead. Calaprice obtained a few samples of this lead and sent it off to Spain, where a lab buried within the Pyrenees tested its radioactivity. It was low: just what Aldo Ianni, the then-director of the Canfranc Underground Laboratory, was hoping for. Now that sunken lead was being offered to any physics laboratory willing to pay 20 euros per kilogram—a fairly high price—for it.

Lead is mined and refined all over the world, but that centuries-old lead, sitting in a shipwreck, has a rare quality. Having sat deep underwater since before the United States of America was born, its natural radioactivity has decayed to a point where it’s no longer spitting out particles. For particle physicists, that makes it exceptionally valuable.

“It’s sort of like gold dust,” Ghag says.

Forget plutonium: Plenty of everyday objects, from ceramics and glass to metals and bananas, are radioactive, to varying degrees. Should the particles from their decay hit the detectors of particle-physics experiments, they could give scientists false positives and dig potholes on the road to scientific discovery. Even the experiments themselves, built from all kinds of metals, have lightly radioactive components.

Just a few inches of lead can shield detectors from all kinds of rogue radiation, and one of the best ways to block sneaky, unwanted particles is to surround them with lead that itself is barely radioactive. The best source of such lead just so happens to be sunken ships, some of which have been corpses near coastal waters for as long as two millennia.

Particle-physics experiments look for the most fundamental building blocks of the cosmos, including dark matter, an as-yet unseen substance that acts like glue within and between galaxies. This ancient lead, then, is helping humanity unlock the secrets of the universe—but obtaining it often presents practical and ethical uncertainties.

Shipwrecked lead belongs to a class of items known as low-background materials, which have very low levels of intrinsic radioactivity. There is no agreed-upon standard for what constitutes a low-background material, but, based on the sensitivity to an experiment’s background radiation, it’s clear what level’s needed, says Alan Duffy, an astrophysicist at the Swinburne University of Technology. “If you’re building a Geiger counter, you need the Geiger counter to not pick up on itself,” he says.

Take steel: It’s an excellent shield from intruding vagabond particles—so much so that Fermilab, a particle-physics and accelerator laboratory in Illinois, has used tons of it in the past few decades to shield its own experiments, says Valerie Higgins, Fermilab’s historian and archivist. That steel frequently came from decommissioned warships, many of which existed around the time of, or served in, the Second World War or the Korean War, including the Astoria, the Roanoke, the Wasp, the Philippine Sea, and the Baltimore.

The timing of those conflicts matters. At 5:29 a.m. on July 16, 1945, the first-ever nuclear-device detonation took place in the Jornada del Muerto desert, in New Mexico. The atomic age had begun, and with each subsequent nuclear fireball, more radioactive fallout was sprinkled over the world.

During the Cold War, that radioactive atmospheric contamination got effortlessly sucked into blast furnaces when steel was made, Duffy says. This infused the final product with radiation, making it unsuitable for many physics experiments.

Test-ban treaties mean the world is less artificially radioactive today, but it is still radioactive enough for particles to sneak into steel. Low-background steel can be made in a sealed environment, often at considerable cost, but otherwise the best source is decommissioned warships, built before the Trinity test created a glassy scar in New Mexico’s earth. Not only is it minimally radioactive, but it’s remarkably cheap.

Yet while steel serves well for all kinds of particle-physics experiments, lead reigns supreme in the search for dark matter.

Dark matter makes up 83 percent of all the stuff in the universe. That clearly makes it worth studying, but scientists can’t currently detect it. In their pursuit of this inconveniently elusive substance, they’ve built all kinds of experiments attempting to either directly detect it or use the presence of other particles to demonstrate its existence. Many of these experiments, from the planned SuperCDMS SNOLAB in Ontario, Canada, to the up-and-running family of detectors within the Canfranc Underground Laboratory in the Pyrenees, are built deep underground—where surface radiation can’t get through and interfere with their detectors.

Being sensitive souls, those detectors still need shielding from their surroundings and the environment. Each dark-matter experiment has a different tolerance for background radiation. To determine it, “you essentially build a virtual detector” to see what shielding materials might be best, Duffy says.

Sometimes a water tank or some plastic is enough to stop particles like neutrons from accidentally hitting the detector, Ghag explains. But blocking gamma rays for some experimental setups can require copper or lead.

Sunken, ancient lead is ideal, not just because its unstable lead-210 isotope would have largely decayed away over the centuries into stable lead-206; the sea has also shielded it from cosmic rays, which can kick-start a material’s radioactivity. Calaprice, who helped design components of several dark-matter experiments, was after that Spanish lead off the shores of New Jersey for these reasons.

That particular load has yet to be harvested, but shopping around such finds is routine. Every now and then, Ghag explains, “some underground lab will say, ‘Hey, there’s an opportunity to buy a load of ancient lead—who’s in?’” Then it gets auctioned off, if the submerged material can be retrieved and there’s sufficient interest from various parties.

In the waters in and around Europe, low-background lead is often found in sunken ships from ancient Roman times. Originally forged into coins, construction materials, and weapons of war, it is now dredged up and sold off to, among others, particle physicists.

Some archaeologists have openly wondered whether it is worth sacrificing archaeological treasure troves in the name of science. Starting in 2010, for instance, the Cryogenic Underground Observatory for Rare Events in Italy obtained hundreds of lead ingots to use for its experiments, all in the hope of solving the long-standing riddle of why matter, not antimatter, dominates the universe. Those ingots came from a Roman vessel off the coast of Sardinia, which sank about 2,000 years ago and has considerable archaeological value. Each of them was inscribed with stamps that reveal their manufacturing history. Although most of the 1,000 ingots extracted from the ship were left intact and made available for study at the National Archaeological Museum in Cagliari, 270 of them were melted down to be used in physics experiments.

In 2013, Elena Perez-Alvaro, then an archaeology graduate student at the University of Birmingham, took up this dilemma. It applied to all venerable underwater shipwrecks rich in low-background material, many of them time capsules of human history. The 2001 Convention on the Protection of the Underwater Cultural Heritage is meant to prevent the skeletal remains of these vessels from being picked clean. But, as Perez-Alvaro pointed out, the convention has a blind spot: Although it demands the protection of sunken sites of cultural heritage from commercial recovery, it says nothing about whether they can be salvaged for scientific use.

In 2015, in a paper she co-authored with Fernando Gonzalez-Zalba of the Hitachi Cambridge Laboratory, Perez-Alvaro concluded that no commercial technique can produce the quality of lead that scientists need for dark-matter experiments. Salvaging ancient lead, therefore, is worth it, in the researchers’ view, but requests should be carefully analyzed on a case-by-case basis to see whether physicists really need lead instead of, say, plastic or steel.

“We have to have rules; we have to have boundaries,” Perez-Alvaro, now the managing director of Licit Cultural Heritage, emphasized. “It’s not just scrap people can dig up.” So far, though, none of the institutions that could impose such regulations has taken up the task, says Gonzalez-Zalba.

Even if an advisory framework for the acquisition and use of ancient lead and other low-background materials does emerge, it won’t necessarily regulate their extraction.

In recent years, warships from the Second World War have vanished off the coasts of Malaysia, Indonesia, and Singapore, illegally ripped apart by salvage divers. Many of these ships were war graves, containing hundreds of corpses. It is possible that some of those divers may have been searching for low-background steel. Buyers may not want to use unethically sourced low-background material, but by the time they receive it, they may have no way to ascertain its provenance.

Although nowhere near as morally repugnant as raided war graves, the origins of lead from culturally significant ancient shipwrecks may be similarly obfuscated. “I guess often, we just don’t care enough to check that,” Ghag says. “It is what it is. We’re more concerned about the cost.”

Gonzalez-Zalba explains that the Romans produced about 88,000 tons of lead each year, and many experiments require only a tiny fraction of this. Scientists, he says, are also increasingly aware of and sensitive to the ethical dilemmas surrounding the extraction of low-background materials.

Particle physicists should keep cultural heritage and the origins of their materials front of mind, Duffy says. But he emphasizes that low-background material is “certainly treated” as a precious resource and not used without consideration.

The real danger, Gonzalez-Zalba suspects, comes from the booming microelectronics industry. Microchips, found in every single computer and smartphone, tend to need low-background lead components. Although the industry could use newly produced lead, he says, manufacturers often chose ancient lead because it’s an order of magnitude cheaper. “This is the application that worries me most, because it’s a commercial application,” Gonzalez-Zalba says. “It’s not an application for the benefit of humankind.”

But with careful consideration, particle physicists can strike an ethical-practical balance. Chasing down mysterious dark matter may feel like a Sisyphean undertaking right up until the moment we find it. But if and when we do, such a discovery will revolutionize our future—and it’s hard to imagine many people arguing that sacrificing a segment of the past in its pursuit wasn’t worthwhile.