Heidelberg is a city in Baden-Württemberg, Germany. As of 2005, 140,000 people live within the city's 109 km² ( 42 miles²) area. The town of Heidelberg is an administrative district of its own. Although not being part of it, the Rhein Neckar Kreis, the rural district, which surrounds the town, has its seat in Heidelberg. The name Heidelberg is an adaptation of Heidelbeerenberg (the German for Blueberry Mountain). Heidelberg lies on the river Neckar at the point where it leaves its narrow, steep valley in the Odenwald to flow into the Rhine valley where, 20 kilometers (12.4 miles) Northwest of Heidelberg, it joins the river Rhine at Mannheim. Heidelberg is part of a densely populated region called Rhein-Neckar-Triangle.