The great Metro fish mystery seems to appear every few months and usually becomes worse in the summer. Last year, according to news reports, it popped up in January, March, and September.

So what’s behind the odor, which has been described as evoking “decaying flesh,” “dead mice,” and “rotting fish?” The Washington Metropolitan Area Transit Authority has named three potential culprits, and basically left it up to you to decide which one is wrinkling your nose.

The first stink suspect: the general grime of a transit system. In September, the Examiner reported that Metro was blaming the smell on innumerable minor factors. The agency also said customers’ noses could become more sensitive due to “environmental, station and train conditions.”

Next up: the sewers. During a different stink episode, Metro blamed the ditches under the tracks of the underground stations. When the ditches dry out, according to Metro, sewer gas escapes, creating the smell. But the stench has also been reported at above-ground stations, where there are no ditches.

While general grossness and the sewer tunnels both figure in as possible explanations, the Keyser Soze of the mystery smell might actually be a set of organic brake pads the agency once ordered for its trains. According to this explanation, popular among online Metro obsessives, the agency’s attempt to go green backfired when new brakes it installed started emitting the dead-fish smell train passengers know so well.

This explanation is backed up by a customer service message from Metro, posted on Metro-tracking blog Unsuck D.C. Metro in September, which blamed the smell on the brake pads. The agency had already decided not to reorder them, according to the customer service employee, but was trying to go through its supply first. Metro later denied that the brake pads were still in use.

Will this mystery ever be solved? Maybe not until the smell comes back. Metro spokesman Dan Stessel claims that the organic brake pads haven’t been used for years. “When I’ve gone down this road in the past, reporting that we don’t use organic brake pads, it usually ends with me being called a liar,” Stessel writes in an email. “So, we tend to not get into olfactory matters these days.”