The blast, which erupted in a vacant lot, was so powerful that it was heard several kilometres (miles) away, toppling nearby disused buildings, shattering windows in residential towers and mangling cars.
![The Senate leadership dissociated itself from this action [Difference Between]](data:image/svg+xml;charset=utf8,%3Csvg%20xmlns%3D'http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg'%20width%3D'100'%20height%3D'100'%20data-ring-placeholder%3D'1'%3E%3C%2Fsvg%3E "The Senate leadership dissociated itself from this action [Difference Between]")
"We were having tea and bwaaaah! It knocked my mother off her stool. It was incredibly loud," said a local woman who gave only her surname, Wu.
The local government said on one of its social media accounts late Sunday that two people were missing following the explosion. Four people were severely injured and 15 others had minor wounds.
Public security officials "have confirmed that the blast's epicentre is a septic tank in an empty field", the statement said.
Methane and hydrogen sulphide -- both highly flammable substances -- can build up in septic tanks.
Authorities had thrown up a cordon around the collapsed buildings and were still investigating the cause of the explosion in Ningbo, a city just south of Shanghai and one of China's largest ports.
An elderly woman said one of her children had been in the area at the time of the blast.
"Yes we live here and one of my children is still inside (the exclusion zone) and hasn't been found yet," she said, declining to give details about the person's age or sex.
She dabbed tears with a tissue and dialled her phone. Asked who she was calling, she said, "my child".
Early Monday, locals were sweeping up piles of debris and glass at the blast site where rubble was strewn across an area several hundred metres (yards) wide.
The city government described the site of the blast as a vacant lot. State media had identified it as an abandoned factory. An AFP reporter said the site appeared to be a very rundown former light industrial area.
Residents said some people from outside Ningbo had been squatting in the area, but others disputed that.
"Most people had left here a long time ago. It was just a wasteland lot," Wu said.
She said most structures in the blast area had already been crumbling for some time, so not all the rubble seen was caused by the explosion.
Another local man, who declined to give his name, said: "It's lucky that more people did not die but no one was living here anymore."
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