Police are treating a large blaze at a former children's soft play centre as arson.
Emergency services were called to Southdown Industrial Estate in Harpenden, near St Albans, at about 21:30 BST on Monday.
Hertfordshire Fire and Rescue Service said it had a number of fire engines on scene, and urged people living nearby to keep their windows closed.
Hertfordshire Police said it "suspected arson", and an investigation was now under way.
The fire service confirmed six fire engines and a command support unit, from nearby fire stations, were sent to the scene.
It said crews worked to put the flames out and it was extinguished by 01:00.
The East of England Ambulance Service, which sent five vehicles to the scene, confirmed there were no casualties.
The Big Space soft play centre closed in 2019 and the building was "abandoned", police added.