<human>:  When did the Bhopal gas tragedy happen?
<bot>: The Bhopal gas tragedy happened on the night of December 3, 1984. It was caused by a leak of methyl isocyanate gas from a Union Carbide pesticide plant in Bhopal, India. The gas leaked into the atmosphere, killing thousands of people and injuring many more. The disaster was the worst industrial accident in history, and it led to a major campaign for worker safety in India.