LOS ANGELES: It smelled like rotten eggs, but the source of what California officials called a “very large and unusual odour event” has been traced to rotten fish.

The stench, which began on Sunday and spread throughout southern California, prompting hundreds of complaints and thousands of jokes, came from the inland lake known as the Salton Sea, air quality officials said on Tuesday.

The smell reached Los Angeles, 240km to the north, astounding experts who at first doubted it was scientifically possible for a smell to travel so far. “I just thought San Diego farted,” said one wag. Official inspectors confirmed the culprit was in fact concentrations of gas produced by decaying fish.

“We now have solid evidence that clearly points to the Salton Sea as the source,” Barry Wallerstein, executive officer of the South Coast Air Quality Management District (AQMD), said in a statement.

Massive fish die-offs are common in the 376 sq mile lake, which is itself slowly dying from excessive salinity, but nobody remembered such a powerful stench.

Experts collected air samples, modelled weather patterns and ran computer simulations as part of “odour surveillance” before concluding a huge thunderstorm churned up fetid waters with bacteria and made the stench airborne, where it became trapped by low-hanging clouds and then gusted north on 60mph winds. “No one using the freeways could possibly have travelled so far so fast in southern California,” noted the Mercury News.

The noxious smell, however, zipped through Mecca, Indio and other towns on its way through the Coachella valley and Los Angeles.

“I think we've shown it was theoretically possible,” Sam Atwood, a spokesman for the AQMD, told reporters. “But this is just something we did not expect.”

Other suspects, besides San Diego, included oil refineries, landfills and natural springs, though from the outset a combination of weather and fish was considered likeliest.

“The problem I'm having is the magnitude of the area that was covered by the odour itself,” Andrew Schlange, general manager at the Salton Sea Authority, told reporters before inspectors confirmed their findings. “But I guess it can happen under the right conditions, and we had those conditions, apparently, the other night.”

By arrangement with the Guardian