A toxic spill in a disaster in SimCity 2000 that can occur randomly, but their presence is even more likely if there are a lot of polluting industries and power plants (coal, oil, and gas) in the city.
They tend to flow downhill, but on level terrain, their path is randomly determined. They appear as gray clouds, floating around as they see fit, and sometimes multiply into more clouds. If they touch a zoned building (residential, commercial, or industrial, but not airport or military), it becomes abandoned.
Using police dispatches seem particularly effective at putting an end to them. Sometimes, Captain Hero will appear to clear them away. A similar event in SimCity 3000 Unlimited called toxic cloud can also occur in areas with heavy pollution.