I had read somewhere that the entrance to the tunnel was in the building “with a green roof” at the St. Pauli Landing area. As I looked around for the entrance, I noted that almost all of the buildings in that area have green roofs. I wound up asking at the tourist information center, and the woman brought me outside and pointed to the building in the distance with the domed roof and said that was the tunnel entrance. Please note I haven’t changed the colors in the image – the building’s roof is indeed brown not green. Whoever wrote that guide I read either didn’t go there, or meant to say it was the only building without a green roof in that area. 😉