Jadala was still trying to work out why he had been arrested in the first place. "I asked the interrogator: why am I here? What do you want from me? When I asked where I was, they told me I was in Honolulu. I didn't ask again," he says.

It later dawned on Jadala that he was there because days earlier his brother, Mohammed, and a cousin, Basher, had been arrested while crossing into the West Bank from Jordan. Israel's intelligence service suspected that Mohammed was a member of Hamas. Sameer Jadala now believes he was detained as part of an elaborate psychological game to pressure his brother into talking. Mohammed Jadala, who is still a prisoner, has signed an affidavit alleging he was tortured into a confession. He says he was beaten during his initial interrogation at a regular prison and then moved to 1391. When he asked where he was, the reply was "on the moon".

"They kept me there in a solitary cell for about 67 days. During this period, they continued with the torture, but they used a different method. They did not let me sleep more than two hours a day. When I started to get drowsy, they woke me up by making noise or by throwing water on me. As a result of the torture, they were able to get me to admit to all kinds of offences," he says.

The interrogators brought the brothers together briefly, apparently as a means of letting Mohammed know that Sameer would pay the price if he didn't talk. "They took my brother and cousin to the secret facility and showed me them crying; the interrogators said that they would be tried because of me," Mohammed says.

Probably the first prisoners at Facility 1391 were Lebanese. The prison is part of a military camp that is home to an army intelligence group, Unit

"Major George" was sacked. Dozens of other interrogators signed a petition objecting to his punishment for using methods they said were sanctioned by the authorities.

Another Lebanese prisoner, Ahmed Ali Banjek, was convicted of smuggling a surface-to-air missile into the Israeli-controlled zone of southern Lebanon on the basis of a confession made at 1391. He later told a military court that it had been extracted under torture, including being forced to sit on a stick until it penetrated his anus. The court was persuaded that the confession was not reliable, and released Banjek.

Facility 1391 remained a secret for two decades or more because those delivered to its clutches could be made to disappear. But even amid the severity of occupation, Israel does recognize that Palestinians have rights. Last year, as the army rounded up thousands of Palestinians during the reoccupation of West Bank cities, it ran out of places to interrogate them. Some were delivered to 1391.

A Jerusalem human rights organization, the Centre for the Defence of the Individual (Hamoked), went in search of one man, Muatez Shahin, who was taken from his home by the army a year ago. The military insisted he was not on any of their lists of prisoners. Hamoked petitioned the high court and, after various attempts by the state to block the truth, won an unprecedented admission earlier this year that Shahin had disappeared into the previously unknown   prison. State prosecutors initially told

504, which specialises in interrogation. The unit has a hard reputation, and some of its members have badly blemished records. One has been accused of murder, another of spying. Unit 504's glory days were during Israel's 18-year occupation of southern Lebanon, interrogating captured Hezbollah fighters and running an extensive network of collaborators, some of whom are still being put on trial for their lives by the Lebanese authorities.

In the late 80s, Unit 504 went in search of another kind of prisoner; men who could be held hostage and exchanged for captured Israeli soldiers and airmen. In 1989, the Israelis seized Sheikh Abd al-Karim Obeid, a spiritual leader to Hezbollah. Five years later, they snatched Mustafa Dirani, a leading Shi'ite fighter. Both were taken directly to Facility 1391.

The soldiers who grabbed Obeid also abducted his bodyguards, members of his family and Hashem Fahaf, a young man who happened to be visiting the sheikh to seek his blessing and who found himself locked up for the next 11 years, initially at 1391.

Fahaf was never accused of any crime, but he was refused access to a lawyer and any other contact with the outside world. For the first few years, the Israelis denied they were even holding him. In April 2000, the Israeli supreme court finally ordered Fahaf's release. The government said it had been holding him and another 18 Lebanese as hostages - or "bargaining chips", as Israeli officials prefer to call it - in the hope of winning the release of an airforce navigator, Colonel Ron Arad.

Mustafa Dirani, the primary target of the abductions, had been the head of security in the Shi'ite movement Amal, and held Arad for about two years, at times driving around with the Israeli colonel in the boot of his car. Dirani was questioned for five weeks around the clock. Freed from Facility 1391 eight years later but locked up in another Israeli prison, he filed a lawsuit in the Israeli courts alleging that he was sodomised by his Israeli interrogators. The legal action names a "Major George" who, Dirani alleges, ordered a soldier to rape him. On another occasion, the Lebanese prisoner accuses the major of thrusting a stick up his rectum. Other former prisoners at 1391 have described how they were stripped naked for interrogation, blindfolded and handcuffed, and a stick was pressed against their buttocks as they were threatened with rape.

In its response to the lawsuit, the Israeli government denied Dirani was raped but it confirmed that prisoners were routinely stripped naked for interrogation. However, the state attorney's office later went further and said that "within the framework of a military police investigation the suspicion arose that an interrogator who questioned the complainant threatened to perform a sexual act on the complainant".

the court that the prison was no longer in use. A few weeks later, the state was forced to confess otherwise.

"The circumstances have changed, and the security people have informed us that detainees are currently being held at Facility 1391," prosecutors told the court.

Hamoked's director, Dalia Kerstein, an Israeli, was horrified. "I was shocked to find out there is such a facility. I don't want the country I live in to have such a secret prison," she said. "We're challenging the legality of this place. We're seeking to close it and we're challenging the whole system of interrogation that goes on in the facility and is a byproduct of the fact that this place is secret, including torture.

"The psychological torture is very intense. People have been there for months at a time. I've met five people from different cities across the West Bank, from different organizations, and they all describe the same methods of torture. They're not beating people but there is very strong psychological torture that results in people hallucinating or having breakdowns."

Sameer Jadala was close to breakdown as he was dragged through interrogation after interrogation that seemed to lead nowhere as his inquisitors tried to get him to implicate his brother or to confess to being a member of Hamas. Then his inquisitors offered him the chance to win his freedom with a lie-detector test.

"I said I know beyond doubt that there is nothing on me. I took the test. At the end, they said 'Congratulations, Sameer' and I never saw them again," he says. "During the night I was visited by soldiers. I was blindfolded and had chains on my hands and legs. They put me into the car, covered me in a blanket and I was driven to a court near Jenin.

"First I had to see a doctor, who asked me where I had been. I said: 'I don't know, I really don't know.' The doctor asked the soldier where I had been. The soldier waved his hand in the air as though he were pointing to a distant planet. The doctor stopped asking questions."

Eventually, Jadala was dragged before a judge, who also wanted to know where he had been held. The prosecutor said he didn't know. "The judge wanted to know if I had a lawyer. I asked how I could appoint a lawyer when I didn't even know where I was. There was no way to contact anybody outside," he says.

It wasn't the end of the ordeal. Government lawyers repeatedly asked the military courts to extend his detention on "security grounds" - by a week or two at a time - but never said what it was he was suspected of.

"During one hearing I burst into tears. The judge asked me why I was crying. I said that for 30 days I didn't know where I was, I had no contact with a lawyer, I was transported in a brutal way. The judge finally said they had to come up with some evidence against me or let me go. So they let me go."