Untere Strasse runs parallel to the Hauptstrasse one block to the south, and is where Heidelberg's bar life is concentrated. The street is lined almost continuously with bars, pubs, and small clubs, most of them aimed at the university's 30,000 students. The atmosphere on a Thursday or Friday night — when the week's lectures are done and the weather is warm enough to sit outside — is as lively as anywhere in southern Germany. The bars are small, the beer is cold, and the crowd is genuinely mixed between students, academics, and younger locals.
Vetter's Alt-Heidelberger Brauhaus on Steingasse, just off Untere Strasse, is a brewpub making its own unfiltered Kellerbier (cellar beer) and Weizenbier on the premises. It's a larger and more tourist-friendly operation than some of the smaller bars, but the beer is genuinely good and the courtyard is pleasant in warm weather. Several bars on Untere Strasse itself have no real name in the guide-book sense — they are simply rooms with a bar and a terrace — and choosing by atmosphere as you walk the street is as good a strategy as any.
The strip is busiest from around 10pm to 2am on weekends. It is very close to the Studentenkarzer and the old university buildings, and the combination of historic setting and student energy is one of Heidelberg's most distinctive qualities.