Why This French Lake Turns Pink Every Summer

A perfectly preserved medieval fortress, on the edge of a bright pink lake. It looks like a set from the Barbie film - and yet it's a real place, and it's in France. But what exactly makes a lake turn pink? And how long does it stay pink? I hopped on a TGV to go and have a closer look...

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IMAGE CREDITS
Lake Hiller by Aussie Oc - https://commons.wikimedia.org/wiki/Fi...
Laguna Rosada by Chad Ajamian - https://commons.wikimedia.org/wiki/Fi...
Dunaliella Salina by Mike Dyall-Smith - https://commons.wikimedia.org/wiki/Fi...
Artemia Salina (brine shrimp) by Hans Hillewaert - https://commons.wikimedia.org/wiki/Fi...
Flamingo by Elgollimoh - https://commons.wikimedia.org/wiki/Fi...
Beta-Carotene powder by 102% Yield - https://commons.wikimedia.org/wiki/Fi...
Carrots by domdomegg - https://en.wikipedia.org/wiki/Carrot#...