This is a video of a really remarkable discovery in the City of David portion of Jerusalem (To see a map of Jerusalem during Roman Times click here). The City of David is the section of the city located on the Ophel Ridge that runs south down from the Harem esh Sharif (aka Temple Mount) and is the location of the ancient Jebusite city that David conquered. During David's time the city was confined to this area and it wasn't until the time of Hezekiah, about 300 years later, that the city wall was extended to the west to incorporate the area known today as Mount Zion. This wall has become known as the broad wall and remnants of it can be seen today.
Today the City of David is not located within the walls of the city. The present walls were built in the 16th century AD by the Ottoman Sultan Suleiman the Magnificent. The tunnel shown in the video runs from the southern end of the ridge north toward the Temple Mount.
(N.B. The term "Herodian" refers to King Herod and describes the time from the commencement of his reign in 37 BC until the end of the dynasty he founded in 92 AD.)