The Copenhagen Metro (Danish: Københavns metro) is the longest underground traffic system in Denmark. It is a 20.5 km (12.7 mi) metro system. It was opened between 2002 and 2007, and has two lines, M1 (green) 14.3 km (8.9 mi) and M2 (yellow) 14.2 km, (8.8 mi). M3 and M4 will be made in the future. In 2009, the metro carried 50 million passengers.