Three Mile Island Nuclear Leak
In 1979, a pressure valve in the reactor at the Three Mile Island nuclear power plant in Pennsylvania released radiation-contaminated water when a pressure valve malfunctioned. This caused the buildings to flood while technicians raced to prevent the nuclear core from melting down.
Had there been a meltdown, the radiation would have escaped into the soil and endangered locals. Luckily, the techs were able to shut the core down, but the entire incident, including the process of fixing the valve wound up costing $839.6 million.