Chicago is famous for this annual event: dyeing the Chicago River green.
The tradition started in 1962, when city pollution-control workers used dyes to trace illegal sewage discharges and realized that the green dye might provide a unique way to celebrate the holiday. That year, they released 100 pounds of green vegetable dye into the river—enough to keep it green for a week!
Today, in order to minimize environmental damage, only forty pounds of dye are used. It only made the river green for several hours.