Costa Rica's Turquoise River - A Natural Optical Illusion

That blue water doesn't look too natural to me.

I mean it's a gorgeous blue, but it's a blue that doesn't seem quite right.

Apparently - at least according to this article - there's no copper in the water, so that's not what it's blue.

Instead, the Rio Celeste produces particulates of aluimosilicate of exactly the right size to reflect light in the blue area of the spectrum. Neither tributary contains particles of the right size, but...

[t]here was only one puzzle left to solve, though. If Rio Buena Vista also had an abundance of aluminosilicate, how come its water looked completely transparent, while Rio Celeste appeared to be turquoise? It turned out to be a matter of particle size. Upon analyzing samples from both bodies of water, scientists realized that aluminosilicate particles in Rio Buena Vista measured 184 nanometers (nm), while those in Rio Celeste were much larger at 566 nm

“This increase in size is what causes the scattering of sunlight, such that it occurs principally in the blue region of the visible spectrum. So that’s why we have that spectacular light blue color of the Rio Celeste” said Dr. Max Chavarría Vargas, who lead the scientific investigation into the turquoise waters of Rio Celeste. “It’s one of those quirks of nature where one of the rivers provides mineral material with one size and the other river provides the acidic environment so that those particles grow.”

Who knew that particulate size mattered?