It is known as the “eye of the earth” due to its strong resemblance to an eye with varied shades of blue and turquoise.
The Lake is just 12km from Izvor Cetine (south croatia) and I have to say this natural karst splendor leaves all visitors speechless.
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What makes it so unusual is its base of hydrogen sulphide; although it's safe to swim here, the water gets hotter the deeper you dive. On occasion the lake 'boils' as the salts and hot water bubble to the surface.

What makes it so unusual is its base of hydrogen sulphide; although it's safe to swim here, the water gets hotter the deeper you dive. On occasion the lake 'boils' as the salts and hot water bubble to the surface.
https://mybestplace.com/en/article/the-eye-of-the-earth-croatias-cetina-river-source