The seaside cliffs and billowing green grass of Alderney, a small, windblown island in the English Channel, belie a dark past that is now largely hidden from view. During World War II, the island hosted Sylt, a Nazi concentration camp where hundreds of prisoners were murdered and subjected to brutal conditions—but whose story is all but forgotten today.

Now, reports Megan Gannon for National Geographic, archaeologists have conducted the first systematic study of Sylt in more than 70 years, revealing how the camp changed over time, as well as the torturous treatment endured by its detainees.

German forces took control of Alderney and the rest of the British Channel Islands following the fall of France in June 1940. Beginning in 1942, the Nazis built several labor camps—including Sylt, Norderney and Borkum—on the island.

The island’s prisoners were mostly from Ukraine, Poland, Russia and other Soviet territories, but there were also a significant number of French Jews. In March 1943, Sylt, already known to be the harshest labor camp on Alderney, became a concentration camp run by Adolf Hitler’s SS paramilitary.

This grim transformation into a concentration camp used to hold political prisoners and other perceived enemies of the state saw Sylt expand from housing just a few hundred prisoners to more than 1,000 detainees, writes George Dvorsky for Gizmodo.

When the Germans retreated from Alderney in 1944, they took pains to cover their tracks; most physical traces that remained were easily overtaken by the countryside. British authorities and locals have also proven to be less than eager to emphasize the island’s painful history, according to the paper’s lead author, archaeologist Caroline Sturdy Colls of Staffordshire University. The physical traces of the atrocities committed at Sylt have been both “physically and metaphorically buried,” she tells National Geographic.

The new research, published this week in the journal Antiquity, combines existing historical knowledge, including testimonials from former prisoners, with modern methods such as aerial surveys, ground-penetrating radar and laser mapping to unearth Sylt’s story.

Drawing on these archival and high-tech resources over the course of a decade, researchers created new maps, a 3-D model of the camp and an overview of the site’s architectural evolution. Many of these physical details corroborate harrowing eyewitness accounts that tell of dog attacks, beatings and shootings.

During their research, the historians encountered gruesome recollections such as this one from Francisco Font, a Spanish Republican held at one of the other labor camps on Alderney: While working near Sylt, according to National Geographic, he saw a “man strung up” on the main gate. “On his chest he had a sign on which was written: ‘[F]or stealing bread.’ His body was left hanging like this for four days.”

Sturdy Colls and her colleagues documented the shallow footprint of Sylt’s barracks, which they estimate provided each prisoner with a maximum of 16 square feet of space. The team also uncovered concrete toilets and created virtual renderings of features that were challenging to investigate in person, per National Geographic.

One puzzling discovery was an underground tunnel leading from the commandant’s house to the camp. The researchers couldn’t determine the tunnel’s purpose, but it appears to have been in frequent use. As the researchers theorize in the paper, it may have served as a “space through which women could be taken into a brothel within the villa.”

The Nazis documented a total of 103 deaths at Sylt, but the researchers estimate the death toll for all of the camps on Alderney was at least 700, reports Gizmodo.

Today, the most conspicuous sign of the German occupation of the island is a plaque placed near Sylt’s main gate in 2008 at the behest of former prisoners. In 2017, Alderney barred development at the site, but the question of how Sylt will be remembered in the future is still being decided.

“The work we did was trying to help the stories of the people who suffered be known more widely,” says Sturdy Colls to National Geographic.

Graham McKinley, a legislator with the States of Alderney, hopes to form a committee tasked with determining how best to preserve, study and memorialize Sylt.

“There is still a small group of people who want to put the past behind them and continue without looking into it too much,” McKinley tells National Geographic. “I believe we should be doing a lot more to show the world what actually happened here.”