New York smelled of maple syrup and nobody knew where from

27 October 2005, ten at night. Calls start coming in to 311, the number New Yorkers use to report anything that is not an emergency. There is a sweet smell. It smells of maple syrup, of pancakes, of freshly made waffles. The first calls come from the Upper West Side and from Chelsea, and within an hour there are calls from the Bronx, from Queens and from lower Manhattan.

Same night, half past eleven. The city's Department of Environmental Protection sends teams out to take air samples. Four years after 11 September, a smell nobody can explain in Manhattan is not an anecdote. The instruments measure the air and find nothing dangerous. It is clean and it smells of waffles.

28 October. The smell has gone and there is nothing left to measure. The police commissioner, Raymond Kelly, says it looks like a food substance and that he cannot confirm it. The press christens it the Maple Syrup Event.

The next three years. It comes back half a dozen times. Almost always at night and almost always with a westerly wind. People talk about a burning pancake truck and a syrup factory on fire. None of that explains why the smell turns up in neighbourhoods so far apart at the same time.

Early 2009. The city stops looking for a factory and starts looking at its own call log. Every report to 311 carries two pieces of data nobody had used: an address and a minute. They put them all on a map, hour by hour, and cross them with the direction and speed of the wind at those moments. The reports draw a fan. Followed backwards against the wind, that fan leaves Manhattan, crosses the Hudson and closes on a point in North Bergen, New Jersey.

There, a Frutarom plant, a food flavourings company, was processing fenugreek seed: the legume that gives curry its taste and that is used to manufacture caramel flavourings. The plant kept working for six more years and closed in 2015.

Fenugreek does not imitate the smell of maple syrup: it has it. The two smell the same because they share the same molecule, sotolon, which is the one in charge of the aroma of both.

Sotolon is noticeable in minute amounts, in the order of parts per billion, which is a teaspoon stirred into an Olympic swimming pool. So a seed warehouse on the other side of the river can perfume Manhattan without a single gram going missing from any inventory. Smelling of pancakes breaks no rule, so there was no fine and no closure order. What the city came away with, after three years, was a molecule with a name and an address in New Jersey.