What was the worst human caused disaster?

the meltdown of the chrenobyle(i know that isn't spelled right) nuclear power plant. the area is still uninhabitable and highly radioactive

This answer is just silly:

1) Genocide in China (1948-1976)

2) Genocide in the Soviet Union

3) Genocide in Cambodia

4) Holocaust

Just these four make Chernobyl look like a sunny day in the park.