<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 gas leak at the Union Carbide plant in Bhopal, India. The leak was caused by a combination of factors including a faulty valve and a faulty tank. The leak released more than 20 tons of methyl isocyanate gas, which killed over 2000 people and injured thousands more. It is considered the worst industrial disaster in history.