By John Sudworth
BBC News, Seoul
A few kilometres from the North Korean border, there is a flurry of last-minute activity and a loud rustling noise as handfuls of waterproof leaflets are stuffed into sacks.
There are 10,000 in each bundle, all destined for one of the most information-starved countries on the planet.
The huge balloons carrying leaflets float into the North on southerly winds
They will get there with the help of helium gas, pumped from cylinders to fill giant plastic balloons, the vehicles for this exercise in paper bombardment.
Each bag of leaflets has its own timing mechanism, readied to burst open at different intervals and scatter the cargo over a wide area.
With a loud cheer, the first of the balloons rises up into the bright blue sky of the Korean autumn.
The wind is blowing gently from the south, and it should be over North Korea in a matter of minutes.
"North Korea is a feudal dictatorship hidden behind an iron curtain," says Park Sang-hak, a human rights activist who defected from North Korea in 1999.
"We're sending these flyers across the border to let the people in the North know about the concept of freedom, and to provide factual information about their leader," he says.
The leaflets repeat the claims made in recent news reports that Kim Jong-il is in poor health as the result of a stroke he suffered in August.
It is a highly taboo subject inside the country over which Mr Kim's family has exercised absolute authority for 60 years.
Professor Brian Myers, from South Korea's Dongseo University, believes the personal nature of the leaflets could be one reason why the North is so angry.
"If Kim Jong-il really is incapacitated then the last thing North Korea wants is its people getting the impression that the country is considered weak," he tells me.
"Whether it is smuggled DVDs or music cassettes, they see it all as an effort to undermine the moral fibre of the nation, so when you get direct propaganda making aspersions about Kim Jong-il personally, they react even more allergically to it."
The North is certainly furious.
It has called the leaflet campaign "psychological warfare" and says that it risks provoking military confrontation.
The balloons are pumped up with helium gas
The propaganda war itself is nothing new, but it was meant to have been brought to an end, at least officially.
For decades the two countries used to bombard each other with messages broadcast from giant loudspeakers facing each other along the length of the fortified border.
But they agreed to stop the angry war of words a few years ago, and the loudspeakers fell silent.
The efforts of private citizens in South Korea, however, have continued, and the balloons have been a commonly used method to try to break North Korea's self-imposed information embargo.
But this latest leaflet campaign seems to have touched a particularly raw nerve.
And now the South Korean government, concerned about the rising tension, has asked the activists to stop.
It is a request that is being given short shrift.
"Our leaflets tell North Koreans about some basic and private aspects of the life of Kim Jong-il," Mr Park says.
"You can see it's effective because of the way North Korea is responding... if we stop, we'd be giving in to their blackmail."
It is difficult to know what any authority, in the North or South, can do to stop their campaign.
The South Korean activists are taking advantage of rights of freedoms of movement and expression denied on the other side of the border.
They can launch their balloons from anywhere - all they need is a southerly breeze.
But the more they send, the higher the tension is likely to rise.