Wastewater treatment plants need to clean their sediment filters out somewhere. Often, that somewhere is the closest fast-moving waterway—and Niagara Falls is no exception.

So around 4 p.m. on July 29th—which was, as Buffalo News put it, “a beautiful Saturday afternoon at the height of tourist season”—sightseers, boat-cruisers, and others hanging out around the continent’s most famous waterfall watched as a smelly cloud of black sludge crept into the Niagara River and engulfed the shoreline around the Rainbow Bridge.

As the city’s Water Board Executive Director, Rolfe Porter, later explained, the plant normally discharges its wastewater basins in the spring and fall, when not so many people are around. But the plant was scheduled for upgrades this past Monday, so they decided to risk a summertime cleanout to prepare.

People noticed. “It looked like something out of a sci-fi movie,” helicopter tour operator Pat Proctor told Buffalo News. (In true sci-fi movie fashion, as soon as he noticed the spreading black cloud, he called the mayor, Paul Dyster.) Maid of the Mist cruises also contacted Dyster, this time via tweet:

Porter said that the black cloud was caused by built-up sediment, along with small carbon particles from the filtration system. The bad smell was caused by what you’d expect.

Although Porter says that the discharge was within the limits set by the Department of Environmental Conservation, New York governor Andrew Cuomo has ordered an investigation.

The Water Board has promised to provide better alerts in the future.

Every day, we track down a fleeting wonder—something amazing that’s only happening right now. Have a tip for us? Tell us about it! Send your temporary miracles to cara@atlasobscura.com.