Plumes of thick black smoke could be seen for miles across Milton Keynes when fire broke out in a yard behind New City Heating in Fenny Stratford this afternoon.
Fire crews were called at lunchtime to the blaze, which was so powerful that it melted large plastic storage containers and metal fitting.
The fire is now under control and the owner of the storage yard has yet to be established.
One witness said: "The smoke was horrible, really thick and black. You could smell the melted plastic.
"We're just hoping there wasn't toxic stuff in that yard because the whole of Fenny Stratford has breathed it in."
The cause of the fire is still unknown.