Experts have been working through the night to try to stop toxic water flooding out of a disused mine near Truro.
Pumps are at full capacity at Wheal Jane, but levels there are still rising following heavy rain. It could mean polluted water leaks into the nearby river and Fal Estuary, a marine special area of conservation. The fire brigade has also been called in to help.
In 1992 flooding from the mine turned the estuary into a polluted swamp.