Emergency workers and construction crews in Hungary were today struggling to clear roads and homes hit by a spill of toxic red sludge.
Hundreds of people were evacuated after the disaster, when a gigantic sludge reservoir burst its banks at a metals plant in Ajka, a town 100 miles south-west of Budapest, the capital.
The torrent inundated homes, swept cars off roads and damaged bridges, disgorging an estimated one million cubic metres of toxic waste onto several nearby towns.
Hungarian officials have declared a state of emergency, calling the spill "an ecological disaster" that could threaten the Danube River, one of Europe's great waterways.
At least four people have been killed by the sludge, three were still missing and 120 injured, many with burns.
Meanwhile the European Union warned the spill could also turn into an ecological disaster in other nations.
EU spokesman Joe Hennon said the EU stood ready to help if the disaster took on bigger proportions.
With the Danube flowing through Croatia, Serbia, Romania, Bulgaria, Ukraine and Moldova before spilling into the Black Sea, there is the potential for widespread serious environmental damage.
"We are concerned, not just for the environment in Hungary, but this could potentially cross borders," said Hennon.
In Kolontar, the town nearest to the plant, mayor Karoly Tily said today he cannot give a "reassuring answer" to residents who feared a repeat of Monday's disaster.
A military construction crew was in Kolontar trying to assemble a pontoon bridge across a toxic stream so residents could briefly return to their homes and rescue belongings.
Emergency workers wearing masks and chemical protection gear also rushed to pour 1,000 tons of plaster into the Marcal River in an attempt to bind the sludge and keep it from flowing on to the Danube, 45 miles away.
Named for its bright red colour, the material is a waste product in aluminium production that contains heavy metals and is toxic if ingested.
Erzsebet Veingartner was in her kitchen when the 12 foot-high wave of red slurry hit, sweeping away everything in its path.
"I looked outside and all I saw was the stream swelling like a huge wave," the 61-year-old widow said as she surveyed her backyard, still under six feet of noxious muck.
Dozens of villagers were burned when the caustic material seeped through their clothing. Two women, a young man and a three-year-old child were killed.
Environmental Affairs State Secretary Zoltan Illes called the spill an "ecological catastrophe," and Hungarian Prime Minister Viktor Orban acknowledged that authorities were caught off guard by the disaster. Orban said the alumina plant and reservoir had been inspected only two weeks earlier and no irregularities had been found.
Red sludge is a by-product of the refining of bauxite into alumina, the basic material for manufacturing aluminium. It is common to store treated sludge in ponds where the water eventually evaporates, leaving behind a dried red clay-like soil.
MAL Rt, the Hungarian Aluminum Production and Trade Company that owns the Ajkai plant, said that according to European Union standards, red sludge is not considered hazardous waste.
The company also denied that it should have taken more precautions to shore up the reservoir, a huge structure more than 1,000 feet long and 500 yards wide.
Clearly angered by the company's suggestions that the substance was not hazardous, Interior Minister Sandor Pinter, snapped: "They should take a swim in it and then they'll see."