Firefighters arriving to tackle a huge tyre depot blaze in County Londonderry found four separate fires across the one-acre site, it is understood.
Crews from across Northern Ireland dealt with the fire which broke out at Campsie Industrial Estate in Eglinton at about 2200 BST on Sunday.
It involved up to 50,000 tyres and it is believed it could take until the middle of Tuesday to fully extinguish.
The Fire Service is examining whether the blaze was started deliberately.
Police have appealed for information.
Residents in Eglinton were warned to keep their windows closed as the fire sent a huge plume of smoke into the air.
Fire Service Area Commander Eoin Doyle said that 16 appliances have been pumping thousands of gallons of water onto the fire.
Colleagues from neighbouring County Donegal were also brought in to help with the effort, as firefighters worked through the night to bring the blaze under control.
Local councillor Jim McKeever said that at the fire's height the flames were up to 50ft high.
"There was a plume of black smoke going over the horizon, which is five or six miles away," he said.
"It's only a few months since there was a fire in an industrial estate in similar circumstances in the city (Londonderry) itself."
The fire is beside a warehouse full of waste cardboard and firefighters worked to ensure the fire did not spread to the building.
McClean Road remains closed at its junction with the Clooney Road at the entrance to Campsie Industrial Estate on the Coleraine side.
Motorists must go into the industrial estate on the Derry side by Courtauld Way.