The toxic Hungarian sludge entered the Danube River Thursday as a result of bursting through a metal plant retaining wall on Monday.
Hungarian prime minister, Viktor Orban, said to the BBC that this event is an ecological tragedy.
The toxic sludge burst through the wall of a metal plant in Western Hungary on Monday, which killed four people and injured over 150 people, according to Reuters.
Fears are growing that the sludge could poison the river, according to the BBC.
The alkalinity levels of the sludge entering the Danube are low, according to the BBC, but that does not mean the battle has been won.
The Danube River is the second largest European river.