Wikipedia article: https://en.wikipedia.org/wiki/Bhopal_disaster

TLDR: There is an American chemicals company called Union Carbide, and they had a pesticide factory in Bhopal, India (around the geographical centre of India).

In December 1984, a storage tank at the factory, storing methyl isocyanate (a toxic gas) leaked due to a water leak into the tank (the gas reacts with water and causes a large increase in gas pressure), and the pressure broke the tank and all the gas leaked out. There were three safety devices installed to prevent this sort of problem, and all three were not working due to poor maintenance.

The gas leaked out to the nearby neighbourhoods, injuring more than 500,000 people. About 8,000 died within two weeks (another 8,000 are believed to have died years later due to long-term illnesses from the poisoning).

Union Carbide claims that it was sabotage by an employee, but there is no conclusive evidence whether it happened on its own or because of sabotage. But, Union Carbide had poorly maintained safety systems which could have stopped sabotage if they were properly maintained and functioning.