The Päijänne Water Tunnel, located in Southern Finland, is the world's second longest tunnel (after the Delaware Aqueduct in the USA). It is 120 kilometers (75 mi) long and runs 30–100 meters under the surface in bedrock.
The purpose of the tunnel is to provide fresh water for the million plus people in Southern Finland in the cities of Helsinki, Espoo, Vantaa, Hyvinkää, Järvenpää, Kerava, Kauniainen, Kirkkonummi, Sipoo, and Tuusula.
The tunnel starts at Asikkalanselkä in Lake Päijänne, which is the second largest lake in Finland, with an area of 1,080 square kilometres. The tunnel slopes slightly downhill so that water flows naturally. Water from the southern portion of Lake Päijänne is of rather good quality at the water tunnel intake and is usually drinkable without processing. The tunnel ends at the Silvola reservoir in Vantaa in the Greater Helsinki area.
According to en.wikipedia