The resulting blast sent flames 500 metres into the air and hurled cars 100 metres off a nearby toll road.
Arguments are still raging over who should pay for the disaster, which has left 400 hectares flooded with toxic mud and forced 12,000 people out of their homes.
Many locals feel abandoned by the government - and by the gas exploration company whose drilling in the area is widely thought to have triggered the mud eruption.
At the same time, experts are divided over whether the mudflow can ever be stopped – and whether the estimated $180 million operation to plug the underground eruption is not just money poured into the ground.
"It was very frightening when the gas explosion happened," says 63-year old Sunarsih, a local resident. "The heat was incredible, even here hundreds of meters away from the site. People didn't know what to do. We thought it was the end of the world."
Sunarsih is one of 847 people evacuated from her original village, Besuki, when mudflows swept in back in May. The fiercely hot mud burst up onto the surface from underground reservoirs and buried many neighbouring settlements.
"We watched, my wife and I," says 46-year old businessman Anwar Hariyono, "as the mud filled up our house and our factory. Now all you can see is the roof. We both cried – the children had no idea what to make of it. We were all afraid."
The upsurge is widely thought to have been triggered by exploratory gas well drilling by the company PT Lapindo Brantas, a subsidiary of PT Energi Mega Persada (EMP), Indonesia's second largest publicly listed energy company. EMP is controlled by the family of a senior cabinet minister, Aburizal Bakrie. He is minister of people's welfare.
"The drilling likely induced an underground blowout," says Jakarta-based independent geologist Andang Bachtiar. "This created a mud volcano, as tons of hot mud shot to the surface under great pressure."
The explosion in Sidoarjo sent flames
up to 500 metres into the air
Java, where Sidoarjo lies, is one of the world's most sensitive geological regions, with the island – home to 62% of Indonesia's 250 million strong population – punctuated by a string of massive, active volcanoes.
The island also lies next to a major fault line, which hit Central Java with a giant earthquake and tsunami back in June.
"The land is very unstable in this area," says H Rudi Novtianto, spokesman for the government's National Sidoarjo Mud Disaster Team.
The land has also subsided some five metres since the eruption began – two metres in a matter of moments on Wednesday night, when the gas explosion happened.
Yet this might just be the beginning.
"My surveys suggest that the eventual amount of subsidence will be about 300 metres," says Bachtiar. "We will likely see the creation of a giant crater, much as you can already see in other parts of Java, though those were triggered by earthquakes rather than drilling.
"How long this will all take we don’t know – it could be centuries – but the area should really just be written off."
However, the national team is determined to try and stem the mudflow and has constructed large pools and dykes to store and divert the flow away from further settlements, as well as nearby road and rail lines. Two relief wells have also been drilled to try and plug the upsurge.
"We believe that with these wells we can stop the mud," says Novtianto. "It is very difficult to do this, of course, and we are now trying to strengthen the dykes and build new run off pools before the start of the rainy season."
This is due to begin in just one or two week's time.
'Dykes will burst'
"We are terrified that the rain will just fill up the pools and the dykes will burst, flooding everything with mud," says local farmer Sunoko, whose land has already been turned into a lifeless wasteland by the mudflow.
"We can't sleep at night wondering what if tonight it will happen."
Like many of the 12,000 people evacuated from their original homes to other nearby locations, Sunoko is also angry at the company, Lapindo Brantas.
"They gave us five million Rupiah (about $550) so we could pay rent on a new home – but for how long?" he asks.
"They also pay us 300,000 Rupiah (about $32) a month for food. This is nowhere near enough. They brought a cistern of drinking water for us, but now that's empty and people have to use water polluted by the mud or buy it at 1000 Rupiah a litre.
"Many people have developed skin diseases and the children have respiratory problems. There’s no doctor from the company provided either."
Others say they have not even received this much.
"All I've had from Lapindo is the 300,000 Rupiah a month," says Hariyono. "When you consider that I lost a factory and a home, it's nowhere near enough. We moved to another village nearby and have had to pay all the rent ourselves."
When contacted by Al Jazeera, Lapindo Brantas declined to comment.
Many of those displaced want the company to pay compensation for lost land and ruined homes, so they can move on to other places and start again.
Yet, "Lapindo have promised and promised to pay compensation but it's now six months since this all started and we've seen nothing", says cafe owner Suriyono, who also had to abandon his home.
While the villagers have been waiting, however, EMP has made two moves to try and sell off its stake in Lapindo Brantas.
Aerial photo of the mudflow, distributed
The first attempt, to sell the company to Lythe, another Bakrie Group affiliate, was ruled out of order by the Indonesian capital markets supervisors.
EMP then tried again, recently selling Lapindo to a company called Freehold, registered in the British Virgin Islands.
This controversial sale is currently under investigation too by the capital markets authorities.
Meanwhile, the government has issued a decree saying that the costs of the disaster must be met by Lapindo Brantas. However, this was apparently contradicted by the governor of East Java, Imam Utomo, who told reporters Friday that the East Javan authorities might foot some of the bill.
Santos, an Australian company which has an 18% share in Lapindo Brantas, says that Lapindo had told Santos that the eventual cost of stopping and containing the mudflow would likely be around $180 million.