Aerial video footage has captured the moment that Chicago's river turned bright green to celebrate St Patrick's Day.

The strange tradition stretches back 40 years, with the city's Journeymen Plumbers taking on the task.

Details of exactly how the river is turned the gloriously bright colour remained a closely guarded secret.

In 2003, one organiser said that sharing details would be like “telling where the leprechaun hides its gold.”

However, it has been revealed that the city uses an orange coloured powder that turns green only once it hits the water.

While spectacular, the colour show does not last for long. After around five hours the dye fades away to return the waters back to their normal sludgy blue.