An investigation has been launched by Environment Agency Wales after reports of pollution in the moat of Caerphilly Castle.
Officers from the government agency have found a fuel or oil based substance affecting parts of the moat, and have taken samples to identify it.
They are working to find the source and investigating what action can be taken to contain or remove the pollutant.
There is no current impact on wildlife but there are fumes from the fuel and a slight visible sheen on the moat.
A spokesman from Environment Agency Wales said: “Our main priority is to limit the damage this has on the moat and to remove it if we can. We then need to find what caused this incident.
“The fuel is more than likely to have entered the moat through the drainage system. If anyone has seen anything suspicious in the area, they can report it anonymously on our incident hotline on 0800 80 70 60.”