The Western Wall visible in the plaza is just a fraction of the full retaining wall of Herod's Temple Mount — most of the wall is buried underground, running northward beneath the Muslim Quarter. The tunnel tour reveals the complete length of the wall and the scale of Herod the Great's construction project becomes clear only here, where you can see the enormous foundation stones at their original depth.
The tour runs about 90 minutes underground, passing through the Hasmonean water channel, medieval cisterns, and a vaulted Crusader hall, before emerging via a ladder into the Muslim Quarter near the Via Dolorosa. The closest point to the Holy of Holies (where Jewish tradition places the inner sanctuary of the Temple) is inside the tunnel, not at the outdoor plaza — a chamber here has been used for prayer for centuries. Advance booking is essential; the tunnel visits are by guided tour only.