Something weird is happening in San Francisco. On Wednesday, local authorities received dozens of reports of a foul, rotten-egg-like smell emanating from an unidentified source.

Most of the calls came from coastal neighbourhoods, such as Bayview, the Marina, Outer Richmond, and Sunset. So far, no one has been able to pin down what's caused the sudden olfactory assault, and officials with the Bay Area Air Quality Management District are still investigating.

The whole problem started sometime around 2 AM on Wednesday, 28 December, when calls started to pour into utility companies, such as the Pacific Gas and Electric Company (PG&E) and local fire stations, with concerned residents wondering what could be causing the noxious odour.

"PG&E received 45 calls about a gas-like odour in several different neighbourhoods, according to company spokesman J.D. Guidi," reports Ted Goldberg at KQED.

"The Fire Department got more than 30 calls, according to that agency's spokesman, Jonathan Baxter. Callers described the odour as 'rotten eggs or sulphur,' … The calls initially came from the downtown area and then moved toward the Marina, Sunset, and Richmond neighbourhoods."

The weird thing is that even with so many different companies, the Fire Department, and government agencies such as the Coast Guard launching investigations into the smell, no one has been able to pin down what exactly caused the sudden flare up.

They have, however, started to narrow the search down. For example, the US Coast Guard has reported that there have been no reports of any sort of spill near the coast.

The San Francisco Public Utilities Commission also claims that the smell isn't coming from any of the area's sewer systems, and PG&E claims that their crews have found no evidence of a gas leak in any of the surrounding areas, either.

The weird thing is that San Francisco isn't the only place to suddenly be hit by a cloud of smelly odours in the past week. Residents in Philadelphia had the exact same issue just a few days ago, but officials have already solved the problem there.

"In that case, though, the source was eventually found – it was coming from a refinery across the river that has lost its power and as a result had some 'flaring and odour' problems'," reports Sarah Laskow at Atlas Obscura.

San Francisco also has a refinery nearby, which also flares regularly - a safety precaution factories such as these use to burn off excess gases that cannot be recycled - but officials working there have refuted claims that the smell has anything to do with the plant's actions.

"A spokeswoman for the Chevron Richmond Refinery said the plant had done some flaring around midnight Wednesday, but that their investigation showed it was not the cause of the odour," reports Evan Sernoffsky at SF Gate.

The Bay Area Air Quality Management District thinks otherwise, though, following up with a new report saying that they are now investigating the plant as one of the leading sources of the smell.

Another possible source is the West Contra Costa Sanitary Landfill, which has been known to cause upsetting odours in the past.

"There have been odour complaints the last few months coming from the landfill," Randy Sawyer, the area's chief environmental health and hazardous materials officer, told KQED.

"They have material that is composting and has turned anaerobic. This becomes very odorous."

The good news is that the smell seems to have dissipated now and officials say the air is not harmful. What exactly is causing the odd smell, though, is still a mystery.

Hopefully as the investigation continues, we'll get more of an insight into what the smell was and what can be done to stop it in the future. Until then, it looks like residents might have to plug their noses.