IT is an opening bare bigger than your own front door. But since it was knocked through, secretively, last Monday night, it has taken on gargantuan proportions - serving as the flashpoint for utter collapse of the Israeli Palestinian partnership.
The doorway that Jerusalem's right wing mayor, Mr Ehud Omert, helped to punch, open onto the Jerusalem Old City's Via Doloroso is not the only entrance to the tunnel. Constructed more than 2,000 years ago by the Hasmoneans, who used it to bring water to the second Jewish temple on top of Temple Mount, the tunnel was rediscovered in the 19th century by the pioneering Biblical archaeologists Charles Warren and Edward Robinson, who entered it via its southern end, from what is today the plaza in front of the Western Wall.
When Israel captured the Old City in the 1967 Six Day War, it kept the tunnel closed to all but private visitors at first, but opened it to the public about six years ago.
The 500 yard passage, which runs alongside but not under the Temple Mount, proved popular with tourists but access was hindered by the fact that there was only one entrance. So successive Israeli governments toyed with the idea of opening a northern entry way, turning the tunnel into a thoroughfare.
Mr Eitan Haber, chief aide to the assassinated Israeli prime minister Mr Yitzhak Rabin revealed yesterday that the issue was raised innumerable times during the Rabin 1992-1995 term of office. He considered a walk along the tunnel passage to constitute a veritable journey to the roots of the Jewish, people," according to Mr Haber. But he always refrained from opening that second entrance, because he feared that an angry Arab response would lead to bloodshed.
Because Jerusalem is so bitterly contested a city, and the Temple Mount area its most sensitive point, any attempt to change almost any structure there is guaranteed to raise tensions. But the ancient tunnel has always been a particularly fraught subject, because of its close proximity to the Temple Mount itself, and the Dome of the Rock and al Aqsamosque that sit upon it.
Among Palestinians, and others, it has often been rumoured that Israeli archaeologists have planned or even carried out digs extending under Temple Mount to search for buried treasure, undermine the mosque structures, or simply puncture the sanctity of the site.
The decision to open the second entry was taken by a five man group, including Mr Olmert, and headed by the Israeli Prime Minister, Mr Benjamin Netanyahu. Mr Netanyahu insisted at a press conference yesterday that the hugely violent repercussions of the move could not have been predicted. But his intelligence chiefs did predict them. Mr Netanyahu chose to ignore them.