Tourists at Niagara Falls on Saturday were shocked when water at the base of the falls turned black—accompanied by a very bad smell. "The first thing that came to my mind was, 'Dear God, please don't be an oil leak,'" Pat Proctor, an exec at a company that flies tourists over the falls, tells the AP.

He spotted the black water, which surrounded a dock for the Maid of the Mist tour boats, from a helicopter. He tells the BBC it spread out over a half-mile, "looked menacing and smelt terrible." The Niagara Falls Water Board had an explanation: The Niagara Falls Waste Water Treatment Plant had gotten approval to "discharge sewer gunk," as the Buffalo News puts it, in order to drain a plant basin that was to be upgraded on Monday.

That explains the liquid and the smell, but not the color. The inky hue was the result of the black carbon filters used to clean the water; in flushing the entire basin, the dark sediment at the bottom was released.

"The blackish water contained some accumulated solids and carbon residue within permitted limits and did not include any organic type oils or solvents," the board's statement said.

"The unfortunate odor would be limited to the normal sewer water discharge smell." The board apologized for scaring residents and tourists—the News reports that Niagara Falls State Park was full of them.

State officials say the water board had all the necessary permits, though many businesses noted the timing was far from ideal. "Why the smelly black discharge into Niagara River on very busy tourist weekend?" tweeted Maid of the Mist.

(A very peculiar death at Niagara Falls has left cops perplexed.)

This article originally appeared on Newser: Black Water Shocks Visitors at Niagara Falls