In the 8th century, the city fathers of Salzburg decided that to prevent illness among the population of the city, a freshwater system of canals that could carry a significant amount of fresh mountain water from the Untersberg to the city center was required. Using a variety of construction materials, including Roman gravestones, the oldest surviving and still-functioning water canal system in Middle Europe was constructed. Transporting water from the distant Königssee in Berchtesgaden (Germany), the Alm Canal supplies the city with a critical resource not only for drinking, but for powering water mills and providing fire brigades with water to put out fires. The Alm Canal is only partially visible, primarily on the south side of the city where, today, many young people spend warm afternoons enjoying an ice-cold dip or surfing on the newly-constructed Almkanal-Welle or 'Alm Canal Wave'.
Near the south side of the fortress, the Alm Canal disappears underground through numerous labyrinthine caves for over 370 meters to feed large reservoirs located within the Mönchberg: still carrying out its intended function after more than 1200 years of service. The water wheel that powers the bakery of St. Peters is, in fact, still in use and lies directly next to St. Peter's cemetery and below the lower station of the Castle's funicular lift. In the fall-usually mid-September-the city drains the Alm Canal and one is allowed to join guided tours through the tunnels within the Mönchberg and be amazed at the engineering abilities of Salzburg's Medieval population.