Doesn't exist anymore.
Yet in 1972, it was thriving and bustling with the flower children of a "Golden Era", bereft of war and bombs and anything particularly unpleasant.
(Photo via https://www.facebook.com/Qadiman)
The Beirut Report draws an interesting comparison to the present day site, located at the intersection of Allenby and Wegan streets, where post-war reconstruction plans have completely demolished any sign of an underground passageway.
Anyways I doubt a subsurface walkway is necessary nowadays; the sad truth is that it would most likely be filled with the homeless.
(Photo via The Beirut Report)