“This is what hell must be like.”

The words of French survivor Jean Mialet express, better than any others, the horrors of the underground concentration camp at Mittelbau Dora in which slave labourers were worked to death making Nazi Germany's V2 “wonder weapon” rockets.

From late 1943, thousands of prisoners from dozens of countries toiled in appalling conditions to produce the rockets that rained destruction on London and other cities. Mittelbau Dora, on the outskirts of Nordhausen in east Germany, was established as a top-secret satellite camp of Buchenwald in 1943 after British bombers destroyed the main missile research base at Peenemuende on the Baltic coast. Adolf Hitler hoped the supersonic V2s—the “V” stood for Vergeltung, meaning retaliation—would turn the tide of war back in Germany's favour. An estimated 20,000 prisoners died making them.

Franz Rosenbach is still astonished that he survived. Arrested in Austria because he was a Gypsy (Roma) (Roma) and therefore deemed “racially inferior,” he was sent first to Auschwitz, then Buchenwald and finally, in early 1944, to Mittelbau Dora. He was 15 years old.

“I am still amazed today that anyone survived,” he recalls. “We got almost nothing to eat, a piece of bread, perhaps two or three potatoes. But you know, when you are young, you can take an awful lot. And if you are careful not to attract attention...I always thought this was not the end for me.” Mialet and Rosenbach will be among around 800 survivors at ceremonies at Mittelbau Dora on April 11 marking the 50th anniversary of its liberation by U.S. soldiers.

The tunnels and caves, the entrances to which were blown up by Russian troops in 1948, will be partly reopened to serve as a memorial to the victims.

The V2 was developed by Wernher von Braun, who after World War Two directed the U.S. space programme. In all, around 5,000 V2s were fired from sites along the English Channel, killing thousands of British civilians.

The first 107 prisoners from Buchenwald were shipped to Mittelbau Dora in August 1943 and put to work carving out new tunnels to enlarge an existing storage depot. Within six months, 12,000 prisoners were toiling in dark, unventilated caverns.

Enduring back-breaking labour, malnutrition and disease as well as the random brutality of their guards, they were also exposed to the gas, noise and dust of explosions. By the spring of 1945, the number of prisoners had reached 40,000.

The death toll was horrendous, with nearly 3,000 prisoners dying between October 1943 and March 1944 alone. Most were Russian, French or Polish.

Thousands of others deemed no longer fit to work were sent to other death camps.

“Until the spring of 1994 the prisoners lived underground,” says Angela Fiedermann, a member of staff at the memorial site. “The sanitation was totally inhuman. There were no toilets and there was no water. The temperature was eight or nine degrees Celsius (46-48 Fahrenheit) and humidity was 90 percent. They died like flies.”

Rosenbach, who arrived as accommodation blocks were being built above ground, worked gruelling eight-hour shifts drilling holes in the rock to prepare for blasting.

“When the explosives were set off, prisoners had to start clearing up immediately. There were lots of accidents, people buried alive under rocks and rubble,” he says.

Rene Steenbeke, a retired Belgian army officer, says his worst memories are of the executions on the camp parade ground. “I saw 51 prisoners being hanged, their hands behind their backs, a piece of wood in their mouths, hanged in groups of about 12. They could see their comrades being killed before them and they had to watch.”

By early 1945, Mittelbau Dora was producing around 690 V2s a month. The monthly death toll among prisoners in the first three months of that year averaged 2,000.

Production ground to a halt in March as Allied troops pushed deep into Germany from both east and west. In April, a partial evacuation began, with already weakened prisoners sent on brutal forced marches to other camps which few survived.

Rosenbach managed to escape from a party of around 500 which set off for Oranienburg concentration camp. Only half a dozen of his group arrived. The others died or were murdered by their guards on the way.

Liberation for the survivors came on April 11, when Aurio Pierro, an acting platoon leader from the U.S. 33rd Armoured Regiment, drove his tank up to the gates. They were opened by surprised prisoners, the guards having apparently fled.

Before his unit moved on, Pierro obtained a glimpse of what lurked within when he entered a building on the periphery.

“There were dead bodies there, naked, emaciated, tied hand and foot,” the retired lawyer told Reuters from his home in Massachusetts.

The rocket equipment was spirited away by U.S. troops in June 1945, filling more than 300 railway wagons, and shipped to the United States to help with its space programme.

Today, the grim subterranean passages where the V2s were made are still littered with footwear, tools and eating utensils. Visitors will gain some sense of the cold, damp and sheer awfulness of the place.

Rene Steenbeke hopes they may also reflect on the part Mittelbau Dora played in launching modern space travel.

“Everything that is now in space had its origins here, not in America or Russia,” he says. “This is where a new science started, but it is also where science and death met.”

Sources: The Nizkor Project;

Nordhausen (Dora-Mittelbau): Survivors Commemorate Liberation