Line M11, officially referred to as the M11 Gayrettepe–Istanbul Airport–Halkalı Metro Line, is a line of the Istanbul Metro. As of March 2024, it is operational between Gayrettepe and Arnavutköy Hastane via Istanbul Airport. The line is currently 51.5 km (32.0 mi) long, making it the longest line in the Istanbul Metro. The M11 is entirely underground, making it the second longest continuous subway tunnel in Europe, behind the Moscow Metro's Bolshaya Koltsevaya line.