Up until four years ago, Rio Celeste, a 14-kilometer river in Costa Rica’s Alajuela province, was a complete mystery to scientists, who could not understand why its waters had an unusual turquoise color. And then they realized that it wasn’t turquoise at all.
Theories regarding the turquoise color of Rio Celeste had been circulating in the scientific community for years, but nobody had ever managed to provide enough evidence to solve this natural enigma. Some claimed that the unusual coloring was caused by high levels of copper, but tests revealed that there was no copper in the water, others said that it was due to chemicals like calcium carbonate and sulfur, and some even connected it to the river’s proximity to the Tenorio Volcano. Everyone was so convinced that a mysterious chemical reaction was turning the water turquoise that they never even entertained the possibility of an optical illusion.