Fire crews and community leaders have marked the first anniversary of the Buncefield blast in a series of events.
A leak of petrol from a tank at the oil depot near Hemel Hempstead, Herts, led to an explosion, which devastated the site and surrounding buildings.
A plaque made out of part of a damaged oil tank has been unveiled at nearby Leverstock Green cricket pitch.
And the Maylands Business Park, which saw major damage and losses to firms of £70m, was relaunched in a ceremony.
Nearly all companies have returned to Maylands, home to a workforce of more than 16,500.
Commemorative events started at 0601 GMT - the exact time of the explosion - with speeches by Daniel Zammit, chief executive of Dacorum Borough Council and the mayors of Dacorum and nearby St Albans.
Mayor of Dacorum, Councillor Catherine Appleby, said that after the blasts people had "pulled together all over the community and in all sorts of ways".
"The 11th of December 2005 and the days and weeks that followed brought out the very best in the people of Hemel Hempstead," she said.
Afterwards, Mrs Appleby unveiled a plaque dedicating a newly-planted oak tree to the people of Buncefield.
The tree is the first of a number to be planted as part of a new tree trail in Buncefield which aims to help counter the environmental effects of the explosion.
The massive blast left two people at the depot seriously injured and the smoke plume from the fire, which burned for three days, was visible across the south of England.
Large parts of Buncefield have been destroyed and the site remains out of operation as the Health and Safety Executive and Environment Agency continue an investigation.
Local Conservative MP Mike Penning, who has complained of delays in settling some compensation claims - some 3,000 people are still awaiting payments - used the anniversary to renew his calls for a public inquiry.
Meanwhile, Fire Brigades Union General Secretary Matt Wrack praised the co-operation between crews from across the country.
"These tireless efforts were above and beyond the call of duty while working in very difficult and trying conditions," he said.
"As always, they showed they are willing to do whatever is necessary to protect their communities."
An anniversary service took place at Holy Trinity Church in Leverstock Green on Sunday night.