This was the shocking scene as a stretch of one of Belfast's best-known rivers was turned an alarming colour of red.
The Northern Ireland Environment Agency is investigating after being called to the river in the Boucher Road area yesterday morning when a member of the public posted photographs showing the water having changed colour.
"River Blackstaff looking healthy today..." Twitter user Aaron K wrote online.
A Belfast packaging firm later admitted it was responsible for the pollution spill that caused the Blackstaff to change colour.
Neal McCone, a director at Delta Packaging, said it was a case of "human error" and that the substance posed no risk to health or wildlife.
"It is an accidental discharge of one of our inks which is a water-based, food-safe ink so it poses no threat to the environment."
He said it was a relatively small amount of ink, approximately 20 or 30 litres at the most.
"It doesn't paint a very attractive picture but even now, it has been significantly diluted at this stage."
He said the product did not pose a risk to fish or plant life.
Alliance MLA Anna Lo, chair of the Stormont Environment Committee, said the spill was still worrying.
"This is very bad news for all of us," she said.
"Pollution in our water really affects biodiversity and wildlife. "
Ms Lo says she is always disappointed to learn of rivers being treated without respect.
"It is very easy for people to throw whatever they want, to discard instead of disposing of it properly," she added.
"It does untold damage to our fish and all the species in the river."
Steven Agnew, leader of the Green Party in Northern Ireland, said: "We need to take these types of pollution incidents very seriously.
"We have enforcement regulations to prevent this type of thing happening," he added.
"We need to ensure when there is a breach there is a strong response, the incident is investigated and if the cause can be identified and intent or negligence is shown that enforcement action is taken."
Fears have been raised over the state of Northern Ireland's waterways after almost six pollution incidents a day were reported to, or discovered by, inspectors in 2013.
Figures published in the 2015 Northern Ireland Environmental Statistics Report revealed there were 2,112 water pollution incidents in 2013, of which 1,310 (62%) had an impact on water quality.
The total number of reported incidents increased by 6% compared with the previous year, while the number of substantiated incidents had risen by 11%.
However, the number of substantiated incidents has fallen by 16% since 2001. The Emergency Pollution Hotline number is 0800 807060.
The Blackstaff River flows largely unseen under Belfast. It was formerly an overground river, but much of it was culverted and built over in the late 19th century. It rises on the slopes of Black Mountain and enters the River Lagan a short distance east of Belfast city centre.
While it is largely underground, it can be seen at locations such as the Boucher Road.
Other Belfast rivers, such as the Farset which gave the city its name, have also largely disappeared underground.