(CNN) -- A tide of toxic red mud burst through the walls of a giant storage reservoir at the Ajkai Timfoldgyar alumina refinery on the outskirts of Akja, a small town in the county of Veszprem, Hungary on Monday, October 4.
So far, four people are known to have been killed by the torrent, six people are missing and more than 120 people have been injured by the sludge which inundated the villages of Kolontar, Devecser and Somlovasarhely, around 150 kilometers southwest of the capital, Budapest.
According to Zoltan Illes, Hungary's Environmental Affairs State Secretary, around one million cubic meters of sludge has leaked from the reservoir covering over 40 square kilometers of land.
Illes described the flood as Hungary's worst chemical accident and an "ecological catastrophe."
Why did the reservoir wall break?
There are no definitive answers to this question yet, but debate is filling the vacuum.
Heavy rain in the region in recent weeks may have caused the foundations of the concrete wall of the reservoir to subside, weakening and eventually causing a section to collapse.
But Hungarian Prime Minister Viktor Orban told Reuters Wednesday that human error and not flooding was the likely cause of the spill.
A briefing "Hungarian sludge flood," published Wednesday, on Nature.com speculated whether the reservoir was "large or strong enough to hold the sludge that it was filled with."
Leading Hungarian alumina technologist George Banvolgyi told CNN that his understanding was that the reservoir had a capacity of "between three and five million cubic meters."
Banvolgyi said that for wall to break on such a reservoir was unprecedented.
"I've been involved in this industry for 38 years and I've talked to colleagues at home and abroad and no one has ever seen such a dam failure," he told CNN.
What's in the sludge?
According to MAL Magyar Aluminum -- owners of the Ajkai refinery -- the majority (around 45 percent) of the sludge in the spill contains iron oxide (hence its rusty red color). It also contains aluminum oxide, silicon dioxide, calcium dioxide, titanium oxide and oxygen-bonded sodium oxide.
People coming into contact with the sludge have suffered burns because the sludge has a high pH (alkali) content and can, if breathed in as dust, cause damage to the lungs.
Chris Bayliss, deputy secretary-general of the International Aluminum Institute (IAI) told CNN: "With the rain that's played a part in the breach of the wall there may have been a watering down of the effects. But certainly the residue would have an elevated pH -- above seven."
Herwig Schuster from Greenpeace told CNN that a pH sample taken by the organization in one of the affected towns recorded a pH level of 13. Greenpeace expects to publish further test results on Friday.
Where does it come from?
The sludge is a residue of an industrial refining method called the Bayer process where bauxite (aluminum ore) is dissolved in caustic soda (sodium hydroxide) to produce aluminum oxide.
This fine white granular material can then be smelted to create aluminum metal or used for non-metallurgical purposes which can be applied in a variety of industries including pharmaceuticals and ceramics.
Estimates differ as to how much residue escaped from the reservoir, but on its website MAL Magyar Aluminum says that between two and four percent of the total volume contained in the dam spilt when the wall broke.
How is the mess being cleared up?
A clear up operation is well underway with more than 500 police officers and soldiers on the scene, as well as specialist detection teams to hunt for missing persons.
A Hungarian government spokesman told the BBC that the clear up will cost tens of millions of dollars and will take at least a year to complete.
A protective wall is being built around the damaged area of the reservoir.
Hungarian Interior Minister Sandor Pinter said guards have been posted at the site to give an early warning in case of any new emergency, the Associated Press reported Wednesday.
Hungarian authorities are continuing to pour tons of plaster into the heavily contaminated local river, the Marcal, in an attempt to stop pollution spreading further.
Anna Nagy, a spokeswoman for the Hungarian government said Wednesday that the leak was under control and the River Danube was no longer in danger of being contaminated.
But this hasn't allayed the fears of the European Union, whose financial assistance will likely be required in the clean up operation.
The EU remains concerned the ecological disaster could still affect neighboring nations, and urged Hungarian authorities to ensure the sludge does not reach the Danube river.