The 80 meters long tunnel runs under the Castle Hill and connects the Old Seminary and the Péter Pázmány Catholic University. The Classicist, vaulted tunnel was built by Sándor Rudnay in 1842. It was named after its lack of proper lightning, though this changed during the renovation in 2006.

The tunnel was an important location of the events of the Revolution of 1956 in Esztergom. A crowd of 1000-1500, unarmed people, escorted by three buses tried to cross here but back then it was military territory. The soldiers tried to turn back the people but in the end they had to retreat. The crowd marched through the tunnel but at the other end there was a T-34 tank which shot a hole in one of the buses and a firefight erupted which had many casualties. Today, we can read the names of the victims on a memorial plaque at the entrance of the Dark Gate.