Päijänne Water Tunnel

At 120 kilometers (75 mi), it is the second-longest tunnel in the world, running at a depth of 30–100 metres (100–330 ft) in the bedrock.

From there, the tunnel slopes slightly downhill allowing water to flow naturally with gravity.

Water from the southern portion of Lake Päijänne is of good quality at the tunnel intake, and usually drinkable without processing.

Since the constant low temperature in the deep tunnel ensures high quality during transport, only minimal processing is required before use.

[clarify] The construction of the tunnel started in 1972 and was completed in 1982, at a cost of approximately €200 million (adjusted for inflation).