Saint-Michel-En-Greve, France ? It should have been a perfect day for Vincent Petit, finishing up an afternoon gallop on a wide expanse of beach along a pastel-colored bay. Instead, he and his mount were sucked into a hole of noxious black sludge.

The horse died within seconds, the rider lost consciousness and a dirty secret on the Brittany coast reverberated across France — decaying green algae was fouling some of its best beaches.

A report ordered by the government after the accident found concentrations of hydrogen sulfide gas emitted by the rotting algae were as high as 1,000 parts per million on the beach where the horse died — an amount that “can be fatal in several minutes.”

There had been signs of a crisis for years in this idyllic corner of Brittany. But scaring away tourists was in no one’s interest, including the farming industry — the region’s economic backbone — whose nitrate-packed fertilizers power algae blooms.

So, while tongues wagged, folks whispered and acrimony grew, an official hush prevailed.

It took the death of the horse to bring the problem into the open.

Decaying ulva algae threatens other beaches around France and the world, from the United States to China, experts say. Last year, the Chinese government brought in the army to remove the slimy growths so the Olympic sailing competition could be held.

In Brittany’s Cote d’Armor region, conditions are perfect for its spread — sunlight, shallow waters and flat beaches. Chemical and natural fertilizers like pig excrement, loaded with nitrates and phosphorous, have saturated the land, spilling into rivers and the ocean, feeding the algae that then proliferate.

Harmless while in water, the algae form dangerous gases — notably hydrogen sulfide, with its characteristic rotten-egg smell — when they wash up on land and decay.

A white crust forms and traps the gases, which are released when stepped on or otherwise disturbed. Over time, putrefied algae turns sand into a black silt muck, sometimes containing pockets of poison gas.

While locals are aware of the perils posed by the silt traps that lurk under the sand around streams that feed from the beach into the ocean, Petit did not sense the danger until the ground gave way and he and his horse were sucked into the noxious ooze up to the man’s chest.