Read: When televisions were radioactive

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.