The golden rule of the entertainment industry is that nobody knows where the next hit is coming from. That was certainly true of Serial, an episodic podcast about the aftermath of an obscure, 15-year-old murder which in 2014 gave millions of listeners across the world an old-fashioned water-cooler moment of shared fascination.
Released without fanfare on iTunes and other digital storefronts, Serial was a surprise sensation from the outset and, within months, its 12 episodes had been downloaded 75 million times. For comparison, that's a larger audience than Game of Thrones, Homeland and Mad Men combined. Here was a genuine, nobody-saw-it-coming-phenomenon.
With the debut this month of a hugely-anticipated second season, Serial is set to equal, perhaps surpass, the success of its rookie year. Demand for the first dispatch was so heavy it caused the servers at several podcast providers to crash. Serial has (almost literally) burnt a hole in the internet.
"It is undoubtedly a blockbuster," says Fin Dwyer, creator of the popular Irish History Podcast. "The question is what it means for podcasting in the future. In the past two years, especially, the level of production has increased - the bar is going up all the time."
Serial has proved podcasts can make real money too. Sponsors flocked to back the series as it transitioned from cult curio to mainstream hit. As did fans, whose contributions helped fund the new batch of episodes. "We're seeing brands get very interested [in podcasts] because they see it as a way to have an intimate connection with listeners," Matt Lieber, co-founder of Gimlet Media, a new Brooklyn-based podcasting venture, said.
This is borne out by figures that show podcasts can charge higher rates than online publications or streaming sites - and far more than traditional radio advertising. Serial's earliest sponsor MailChimp, for instance, is said to have paid up to $40 "per thousand impressions" (the standard metric for web advertising). Network television streams, in contrast, bring in about $5 to $20 per thousand impressions while the return for standard pop-up web ads is between $1 and $20.
This success has been duly noted, with the corporate world turning its attention to podcasting in earnest. In November, the science fiction podcast The Message reached number one on the iTunes podcasts charts - ahead of heavyweights such as This American Life (of which Serial was a spinoff) and comedian Marc Maron's confessional interview show, WTF.
However, The Message wasn't the work of plucky underdogs. It was created by the marketing department of General Electric, in conjunction with Panoply, a podcast network owned by digital site Slate (and contained a hard to miss plug for GE's ultrasound technology). "It's a reinvigoration of the oldest form of communications," said Levi Slavin, of BBDO New York, the advertising agency that collaborated on the project with GE.
"You've got this captive audience commuting or on a road trip or stuck in traffic and they're choosing between genuinely entertaining personalised experiences through audio."
Yet though the emergence of podcasting as a financially viable medium is good news for listeners, for podcasters further down the food-chain the outlook is mixed. To be a plucky striver with a message for the world is no longer enough, says Fin Dwyer.
"It is much more difficult for independent podcasts to break in," he says. "If you go to the iTunes charts at the moment - it is dominated by large organizations - 2fm, NPR. Shows that have a very substantial apparatus behind them.
"My podcast is in the top 20 or top 30. Were I to start now could I do the same? I don't know. I have an established audience that allows me to get the nudge on someone coming in new."
Podcasting also suffers from the curse of the internet - anyone can record an hour of audio and put it online.
"Podcasts have plusses and minuses in that they do make it easy for creative people to make stuff and not have to worry about a station manager's bias or lack of interest in upsetting schedules," says Jho Harris of All Points West, a podcasts network based in Sligo.
"On the minus side it also allows people without creative skills to make podcasts and the market is getting flooded."
For the producers of Serial a more prosaic problem loomed: how to capture lightning in a bottle two years running? Their solution, it seems, has been not to try. Where Serial year one focused on the killing of Hae Min Lee and her boyfriend Adnan Syed's conviction for her murder, this year the subject matter is the far better known Bowe Bergdahl, a US soldier captured by the Taliban apparently after wandering away from his post while on guard duty. It's a fascinating story - just one already familiar to anyone who has closely followed Middle Eastern affairs.
"Our first season was about a murder case few people had heard about. Season two is a story a gazillion people have heard about: the story of Bowe Bergdahl," acknowledged Serial presenter Sarah Koenig.
Judging by the stampede to download the first episode, fears it would flop are unfounded. What's harder to predict is whether the spirit of amateur derring-do that has until now characterised podcasting can survive when real money is at stake. With so much cash suddenly to play for, it seems unlikely.