Liron T.
Yelp
The old Elbtunnel is a relic from a not-too-distant past, when men were men and tunnels were... tunnels. This tunnel was built in the 1910s, back in the day when tunnels weren't built at gradual slope, but were dug straight down, across and back up again. If you're on foot, a flight of stairs or a modern elevator will take you down, if you're in a vehicle, the large shaft has three separate elevators for cars.
The tunnel itself is only a few hundred meters long, but is a lot of fun to walk through. Interesting detailing of marine animals (and a boot) can be found on the walls, and interesting bits of information are tacked up as well, such as a sign explaining that the tunnel was actually deepened (as in, relocated and placed deeper underground) in the 1980's. The acoustic quality of the tunnel is unique - we were down there with musicians on more than one occasion (a few guitar players and also 3 members of Sweden's Gothenburg Chamber Choir) and were treated to delightful improvised performances.
Upon exiting the tunnel from the other side (assuming you went in from Landungsbrücken) is a nice place to look over the Hamburg waterfront and most of the city's landmarks.
In the winter, it is several degrees colder in the tunnel than outside, so make sure to dress warmly.
When showing people around the city, I try to make sure not to skip the old Elbtunnel. I think it's one of the interesting and unique pieces of architecture Hamburg has to offer.
Here's a short clip I took while going through the tunnel with my younger brother who came for a visit in Hamburg.
Video link: http://www.youtube.com/watch?v=KhHr-YzKo74