The colour is incredible. The middle of the pool is the deep dark blue of a midnight sky, and around the edges it fades to bright turquoise, like the rim of a peacock feather. The shape is almost a perfect circle. Looking at it from the wooden walkway above feels a bit like looking into another world.
Divers have tried to find the bottom of the Blue Eye and so far nobody has managed. The water is around 50 metres deep at least - and probably more. Beneath that, an underground river keeps pushing fresh, very cold water up to the surface every second. The water comes out at the same chilly temperature in summer and winter.
Because the water is so cold (about 10°C), most visitors only put their feet in. The pool then becomes a stream that flows through a forest of beech trees and joins a bigger river further down. The whole place is a protected nature reserve, so it stays as wild as possible.
Local stories say a dragon used to live in the spring and that the dark blue centre is its eye. That isn't true, of course - the dark colour comes from the deep water - but it makes a good campfire story when families visit on a summer day.