A huge fire sent thick plumes of smoke billowing into the air on Saturday.
A neighbour said she heard an explosion moments before thick, black clouds rose to the skies.
Approximately 200 tonnes of scrap metal was on fire.
It happened on a scrapyard in Oakridge Lane, Aldenham, at around 2pm on Saturday, the Watford Observer reported on Saturday.
Fire crews spent almost 24 hours battling the blaze, which was out by 11.40am the next morning.
Planes were forced to fly through a dark cloud of ash as fire crews raced to the scene.
People were told to keep their doors and windows shut as the smoke choked the area.
Thick plumes of smoke could be seen from Watford, Borehamwood, Bricket Wood and Radlett.
It took fire crews all night to extinguish the flames.
One witness said: "It looks really serious. The ash cloud is so high planes are flying through it."
There's a bad #fire going on between #Radlett and #Watford at the moment #dronephotography #aerialphotography pic.twitter.com/I8nREHSvWg
— Helixcopters (@Helixcopters_UK) September 29, 2018
Does not look nor sound too good this. Fire been raging a good 40+ mins . #radlett pic.twitter.com/CD2Qy1csEf
— Jason G (@SirLicht) September 29, 2018
Black Smoke fire in somewhere near Radlett Road Watford pic.twitter.com/WoG7ujYwNh
— 999 Herts News 🚨 (@999herts) September 29, 2018
Helping supply water to extinguish 2000 tons of scrap metal well alight in Aldenham this afternoon/evening/night!! The smoke could be seen across London apparently! @HVPCapability @HertsFRSControl @JDSemps #weneededabiggerpump pic.twitter.com/80WMNFwN0a
— Welwyn Garden City Green Watch (@WGCGreenWatch) September 29, 2018
Further to our previous tweets regarding the SCRAPYARD FIRE in ALDENHAM - if you are in close proximity to the area of Aldenham and Radlett - please ensure your windows and doors are closed to avoid inhaling the smoke fumes. #Aldenham #Radlett #FireSafety #Smoke
— Herts Fire Control (@HertsFRSControl) September 29, 20
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