The news of the discovery of a sunken Portuguese slaving ship off the coast of South Africa is both exciting and deeply moving.

According to the Washington Post, in 1794, the São José-Paquete de Africa carried 400 slaves bought in Mozambique and headed for Brazil’s sugar cane plantations. Half of the slaves died. The survivors, valuable property, were resold.

Iron blocks of ballast

Iron blocks of ballast

Among the objects discovered are shackles, chains and the iron blocks used as ballast, since human bodies were too light to properly balance and weight the ship. Some of the recovered artifacts will be on display when the Smithsonian’s National Museum of African-American History and Culture opens on the National Mall in 2016.

None of these artifacts are precious or even beautiful. They’re the everyday implements of what was once a booming trade. But the fact that they are made of metal and pulled from the sea made me think of another deeply moving memorial. At Chile’s Villa Grimaldi, once a clandestine torture center, little remains of the cells or implements used to detain, torture and kill thousands of Chileans during the dirty war, from 1973-1990.

Since, Villa Grimaldi has become a peace park. At the foothills of the Andes, it’s a spectacular place, where memory seems to rest in the shadows of the araucaria trees. I’ve visited several times, always with a sense of calm and the faint chill of what happened there, still palpable.

Button encrusted in an iron rail found in the ocean off the port of Quintero, Chile

Button encrusted in an iron rail found in the ocean off the port of Quintero, Chile

In one of the displays is a piece of iron rail. On it is encrusted a single plastic button, from a person’s shirt. This is mute evidence of what was once common practice. Detainees who no longer served a purpose were often drugged then flown out over the Pacific. Their bodies, lashed to rails, were then dumped into the water. For years, reports of this were purely anecdotal, without physical evidence. Then in 2004, off the port of Quintero, the sea gave up a disturbing prize: a single while button encrusted in an iron rail.

The button is so simple, so unremarkable. But everyone I know who’s seen it has come away with a better sense of the true horror of what happened. Filmmaker Patricio Guzman even made a documentary inspired by the artifact, called The Pearl Button.

For me, that’s what these iron blocks do. So simple, yet so horrifying — the literal weight of the slaves transported from their homes for sale.

“If water has a memory,” said poet Raul Zurita, “then it also remembers that.”

And so does iron.