Seoul, South Korea | Seoul office worker Park Sun-Min constantly checks his smartphone -- trawling for updates on an insect apocalypse, ghost soldiers haunting the inter-Korean border, and a supermarket worker’s struggle to form a trade union.

Along with millions of other South Koreans, Park is, by his own admission, irrevocably hooked on the vast network of varied Internet-based comic strips -- or "webtoons" -- available through his mobile.   

"I read four to five a day and more than 30 a week ... I sometimes see them at work and keep reading them on holiday -- even overseas," the 30-year-old said.   

The genre is a growing cultural force in South Korea, supported by an ultra-fast Internet and smartphone-crazy populace, and fuelled by a small army of young, creative, tech-savvy graphic artists. 

Most webtoon serials are published on major Internet portals free of charge and once or twice a week. They cover pretty much every genre, from romantic comedies to horror, via historical epics and crime.

Their popularity has drawn the attention of the wider entertainment industry, and top rated webtoons have been successfully adapted into TV dramas, films, online games -- even musicals.

According to Digieco, a Seoul-based technology think tank, the market for webtoons, and their "derivatives," is currently valued at around 420 billion won ($368 million) and is expected to more than double to 880 billion won by 2018.

- Steep earning curve -

A recent example of the sort of stellar trajectory a webtoon can take was provided by "Misaeng" (or "Incomplete Life")-- a highly-acclaimed series about a young part-time worker trying to survive South Korea's cutthroat corporate culture.

The twice-weekly comic built up an Internet readership of one million, and the series was collated in a book version that sold two million copies.

A TV drama spin-off was a major hit last year, and the final accolade came when the government named a new piece of legislation to help part-time workers after the webtoon's main character.  

"I think this is a distinctive genre ...and the market is exploding at a mind-blowing pace," said Cha Jung-Yoon, a spokesman for Naver -- Seoul's top Internet portal. 

South Korea had a traditional comics industry which all but collapsed during the 1997-98 Asian financial crisis that drove many publishers into bankruptcy. 

But the Internet opened a new door. Naver launched a dedicated webtoon section in 2005, commissioning three artists whose work attracted 10,000 views a day.

The section now boasts more than 220 commissioned artists and 7.5 million daily views, Cha said, adding that 75 percent of readers are aged 20 or older.

- 'A whole new genre' -

Most webtoons are created digitally, often in long-strip format for scroll-down viewing on computers or smartphones.

Many contain moving, flashing or 3D images as well as sound effects and background music. Some even make the smartphone vibrate when readers scroll to a certain scene.

"Webtoons are not simply scanned versions of print comics. It's a whole new, different genre tailored for the Internet age," said Kim Suk, senior researcher at state-run Korea Creative Content Agency.

"The introduction of smartphones in 2009 was a watershed moment for webtoons ... it really fuelled their growth," Kim said. 

More than 80 percent of the South's 51 million population own smartphones, allowing fans to read webtoons anywhere, and they have become particularly popular with commuters.

Seok-Woo became a full time webtoon artist after a series he devised -- a psychological thriller about school bullying -- won a 2007 competition to publish a regular series on Naver.

The 32-year-old, who writes under his given name, grew up in a family that moved a lot when he was a teenager, leaving him feeling friendless and isolated.

"Many artists blend their own life experiences into the story and that often resonates well with readers," he told AFP.

Action heroes are relatively rare -- often the most popular webtoons are those dealing with issues like poverty, cyber bullying, suicide, youth unemployment, and domestic violence.

- Webtoon A-listers -

With fan bases growing, dozens of webtoon creators earn six-figure US dollar incomes, made up of fees from Internet portals and advertisers, as well as licensing contracts for TV shows, films and other adaptations.

A handful of A-listers appear on TV talk shows, attend mass signing events with fans and give motivational speeches to cartooning hopefuls.

But the competition is intense.

Hundreds post their webtoons online each week, hoping to attract a big enough fan base to push them into the big time -- a contract with one of the major Internet portals.  

"It's actually quite nerve-racking because readers rate each episode, with the number of clicks and readers' feedback shown right in front of your face each week," Seok-Woo said. 

His webtoon -- a teenage romance called "Orange Marmalade" -- became a hit and was adapted for television.

"It's a bit surreal how the webtoon has become a next big thing so quickly," he said after returning from a book-signing tour in Indonesia, where a translated version of "Orange Marmalade" has garnered a whole new set of fans.

The most popular South Korean webtoons are now being offered in Internet and mobile platforms in China and Japan. 

And, after a barrage of requests from foreign fans, Naver recently created a global service offering hit webtoons in English, Chinese, Thai and Indonesian.