For more than 60 years Chicago city has been dying its river green with a "secret formula" to mark its St Patrick's Day parade and festivities, celebrated by more than 2 million people.
The tradition of dying the river began in 1962, when the local plumbers union, who still hold the honour, dyed the river green, lasting up to a month. Nowadays, it takes place on the Saturday before and lasts only a few hours.
The river was dyed green on March 11 this year, when the St Patrick's Day parade took place.
