Two workers suffered burns in an early hours explosion at Tata's biggest steelworks plant in the UK.
Residents living near the Port Talbot plant heard a "massive" blast shortly after 03:30 BST.
Images and footage posted on social media showed a huge mushroom cloud and plumes of smoke rising above the plant.
Tata Steel said the explosion came from a train carrying molten metal. It said the workers had received treatment for minor injuries.
"The spillage led to a number of fires which were extinguished by our own emergency services supported by members of the Mid and West Wales Fire and Rescue Service," it said in a statement.
BBC Wales understands the explosion happened on a stretch of railway track between the engineering shops and the locomotive repair shops.
Stephen Davies from the Unite union believed a train - carrying a "torpedo" which holds the molten metal - derailed, leading to the metal coming into contact with cold water.
He said: "They [the men] were both on the train. One of them is close to retirement and the other is younger."
One of them had a burn to the head and the other to the chest, he added.
The site reopened by 07:00 although production at blast furnaces four and five have been halted until all checks had been completed, Mr Davies said.
Lance Davies, 36, who lives at the highest point in the town overlooking it and the steelworks, said it was a "miracle" no-one was killed.
"I was woken up this morning by what sounded like thunder. I went to the window and could see the results of the first explosion, then I called my partner and saw the other two," he added.
"That was all you could see - it was like a scene from Independence Day - a big ball of flame, followed by a massive mushroom of smoke. It was unbelievable."
Sharon Freeguard, a Neath Port Talbot councillor, said: "I thought I heard about two to three explosions. It was extremely frightening.
"The house just shook, so it was quite alarming. Once it was over, I did wonder what it was and just did think about the works.
"We're just all relieved that there are no fatalities."
Local resident Craig Williams said he heard "an almighty bang", adding: "It's very unusual to hear something of that magnitude.
"The house shook a little. It's not something we are accustomed to."
Mr Williams said Port Talbot "revolves around this plant" and it helps to "bring the community together".
The steelworks is the largest in the UK and employs more than 4,000 people.
Helicopters were circling overhead and South Wales Police said it received "numerous calls" shortly after 03:30 "reporting an explosion" which, they said, had caused damage to some buildings on the site.
Mid and West Wales Fire and Rescue Service said 10 appliances it sent to the scene had left by 08:40. The Health and Safety Executive added it would be making contact with the emergency services.
The explosion was heard as far away as Bridgend, 14 miles (22km) from the blast, and the National Police Air Service's St Athan wing confirmed a helicopter had been scrambled to the scene.
Aberavon MP Stephen Kinnock said safety at the plant had improved "massively" since 2001, when three workers were killed in a blast.
He said he had spoken with Tata bosses on Friday and was told a review was under way into the cause.
"We've got to understand why it happened and make sure it doesn't happen again," he said.
I thought another blast furnace went. I hope nobody was hurt. The steel works is the heart of this town, our workers Notre Dame. I can see the fire now. Who will rebuild this? TATA? Who will rebuild our town? We’ve been waiting for years. #PortTalbot pic.twitter.com/27MrSrS2fE— Damian Healy (@DamianHealy) April 26, 2019
Aberavon AM David Rees has also requested a meeting with the steel firm.
A spokesman for union Community said: "It is important that all appropriate procedures are followed now to ensure lessons are learnt and any necessary changes are implemented."
In an updated statement on Friday evening, Tata Steel said it was in contact with its two employees who had "suffered minor injuries and received treatment".
"We are investigating the cause of the incident and are working closely with relevant agencies including the Health and Safety Executive," it said.
"Customer orders have been unaffected and we continue to supply steel to both customers and our downstream businesses by using existing stocks.
"Meanwhile, we are working towards bringing at least one of the two blast furnaces back online."
In a further message on its Facebook page, Tata Steel warned residents the work "may result in some noise and steam from the furnace".
"This is completely normal for this type of operation," it said.