Prisoner X, a 34-year-old Australian Jew named Ben Zygier who appears to have been involved with the Mossad, Israel's foreign intelligence service, was imprisoned by Israel in February 2010 for what has been described as "grave offenses."
These haven't been explained. What is known is that he was incarcerated in a highly secure wing of Ramle prison near Tel Aviv, possibly because he was about to spill the beans on Mossad's covert operations to Australia's intelligence service, or to the media.
The Australians apparently suspected he was involved in securing Australian passports for Mossad hit teams, possibly including the January 2010 assassination of a senior Hamas leader in Dubai as he sought to arrange an arms shipment with the Iranians.
The 26-member Mossad hit team that killed the Hamas operative Mahmoud al-Mabhouh, first drugging him in his Dubai hotel room them smothering him to death, used Australian, British, Irish and German passports. All escaped after the assassination.
Where Zygier fitted into all this remains unexplained. So does the reason for throwing him into a highly secure cell, originally built to accommodate the Jewish fanatic who assassinated Yitzhak Rabin in November 1995 for his peace treaty with Yasser Arafat.
Zygier was held there incommunicado until he hanged himself Dec. 15, 2010, in a cell under round-the-clock electronic surveillance.
But other prisoners have vanished into this Israeli black hole over the years.
One of them was Marcus Klingberg, an Israeli chemical weapons expert and onetime deputy director of the top secret Israel Institute for Biological Research outside Tel Aviv, who was actually a Soviet spy.
After Israel's General Security Service got onto him in 1983, he was lured to an undisclosed location after being told he was going to Singapore to investigate a fire at a chemical plant there.
He was kidnapped by Israeli agents, tortured and imprisoned. His trial was held in camera and even his wife was banned from attending the proceedings involving the state against "X," or even telling anyone he had been arrested.
After a secret trial under a false identity so no one would know of his 20 years of treason or the highly sensitive work he did at the institute, reportedly the hub of Israel's chemical and biological warfare program, he was given a 20-year sentence.
The Polish-born Klingberg, who lost his entire family in the Holocaust, was locked up in Ashkelon prison south of Tel Aviv under the phony name of Abraham Greenberg. For the next decade, he was held in solitary confinement and military censors ensured that the Israeli media never mentioned his name. To all intents and purposes, he had ceased to exist.
He was released in October 1998, at age 81, after serving 16 years but was kept under constant surveillance because he was considered a threat to state security. He lived alone in a fourth floor apartment with barred windows and allowed out for only a few hours every day.
He was eventually released Jan. 18, 2003, and went to live in France with his daughter Sylvia and her family.
Another Prisoner X was Mordechai "Motke" Kedar, probably the model for others given that tag. The Polish-born Kedar was recruited by Israeli Military Intelligence, known as Aman, in 1956 for a mission in Egypt before Israel's thrust into Sinai later that year.
He was arrested, charged with unspecified offenses while a member of Aman's Unit 131, and incarcerated in total isolation for 18 months in Ramle prison, where even the guards did not know who he was or why he was there. Shades of Zygier.
Kedar's trial before a military court was held behind closed doors. He was sentenced to 20 years in prison.
To this day, the Israeli government has maintained an absolute silence about his case.
Nothing is known about what Zygier was charged with but he reportedly ran a shell company in Italy exporting telecommunications equipment to Iran as part of an intelligence operation.
That may be one of the secret operations the Israelis feared he was about to expose.