Racing to build a bomb that would underpin its status as a major Cold War power, France chose this barren spot in what was then a French colony to carry out nuclear explosions before a newly independent Algeria called a halt to the tests.
Nearly half a century later, local people - backed by Algeria's government - say the tests left a legacy of environmental devastation and health problems, and are demanding that Paris issue an apology and pay compensation.
The issue has become a symbol of the tension between Algeria and France. Algeria is angry that Paris has not offered a broader apology for what it sees as France's colonial crimes, and relations hit a new low this week when Algeria accused France of failing to back its fight against Al-Qaeda militants.
Hussein Dakhal, who lives in a village near In Ekker mountain, is now 83. He remembers when, on May 1, 1962, the French conducted a test codenamed "Beryl.” It went wrong, letting radioactive material escape from inside the mountain.
"I heard the explosion. Since then, life has changed for us ... unknown diseases and health problems started to emerge," Dakhal said as he stood near the foot of the mountain, about 2,000 km (1,250 miles) south of the Algerian capital.
After a war that killed hundreds of thousands of people, Algeria won independence in 1962. But under the treaty signed by French President Charles de Gaulle ending rule from Paris, the tests were allowed to go on until 1966.
Starting in 1961, France conducted 17 nuclear test explosions at In Ekker, where it carried out underground blasts inside the mountain, and in the Reggane region of the Sahara desert, where it conducted surface explosions.
El Hamel Omar, president of a local association that tries to alert people to the dangers of contamination said: "Cancer killed a lot of people in the region, but very often the victims as well as their parents did not know they were ill.
"Infertility, cataracts are also problems that faced victims of nuclear tests in the region. Remember it is a remote area, and access to medical treatment is for many a luxury they simply can't afford," El Hamel said.
"Our job is to shed light on the causes of death of people in the region. We need help from the medical and scientific communities, but the government is backing us." Dressed in the loose gown and headdress traditionally worn by Sahara desert tribes, Dakhal has no doubt about what he sees as France's responsibility. "I want an apology and compensation," he told Reuters.
France's Defense Ministry says studies at the time showed that the levels of radiation experienced by local people were well below admissible levels and that environmental contamination was not bad enough to be a health threat.
Today, decaying signs reading "Danger" in French and Arabic stand around the mountain, and abandoned pieces of equipment - a section of rail, metal barrels and tin cans and even a soldier's boot - lie strewn around the site.
Officials carried Geiger counters to measure radioactivity as they showed reporters and campaigners around the site last week on a visit organized by the Algerian Ministry for Mujahidines, or veterans of the war of independence.
According to Algerian data, radiation in some areas near the test sites is 20 times higher than the norm. "Do not stay more than 10 minutes. It could be dangerous," one scientist at In Ekker told the visitors.
Algerian officials say France is refusing to give them access to archives about the tests, leaving them in the dark about the extent of the threat from radiation and preventing them from taking effective measures to contain it.
"The region has been irradiated. We need information about where irradiated stuff has been buried, this is why it is vital to obtain archives from France," said Roland Desbordes, head of an independent French nuclear watchdog.
"I do not understand why France is against the principle of delivering the archives to Algeria," he told Reuters.
Not all those who say they were victims of the testing are Algerian. A French newspaper, citing confidential documents, reported this month that France deliberately exposed its soldiers to the blasts to study the effects on humans.
Michel Dessoubrais, who at the time was in the French military, said he was one of those unwitting guinea pigs.
"I was there. I saw the huge mushroom cloud on May 1, 1962 at 11 a.m. Our bosses abandoned us at the site," Dessoubrais said last week on a return visit to the mountain. "What happened at In Ekker is not acceptable."
