The Alter Elbtunnel is a cool and free place to check out in Hamburg. It was the original tunnel…read morebuilt for connecting two parts of the city across the Elbe, but has since been superseded by newer tunnels and bridges. For this reason, I don't think cars a very common sight in the tunnel, although they can go through it for a fee.
There are very old and interesting lifts going down to the tunnel, which pedestrians and cyclists can also use, but I preferred to use the stairs, from where I could look at the lift's mechanisms. I'm interested in that kind of thing.
The tunnel is long and it's straight. It was a fun experience to walk through it and then come up at the other side, with a tremendous view of Hamburg. From the other side, you can see St. Pauli, the harbour, HafenCity, and lots of church spires as well. It's worth it just for the view. Panoramic Photo Op!