Built using 3D printers and components sourced online and sometimes equipped with GPS-tracking, these so-called "smart balloons" can cost up to $1,000 each.
Once or twice a month from spring to autumn, when favourable winds blow north, the secretive group flies the balloons - mostly under the cover of darkness. The aim is to drop cargoes deeper into North Korea, including over the capital Pyongyang, with the longer ranges now possible. One balloon has flown as far as China.
"Our smart balloons are expensive but we think they're a hundred times more powerful than balloons flown by other groups," said one member of the group which is called "The Committee for Reform and Opening up of Joson". Joson is another word for North Korea.
The group, which has some 30 core members and is funded by members' own finances as well as donations, has not previously detailed its activities to media.
Balloon tactics have taken centre stage in the frosty relationship between the two Koreas since late last month. North Korea, in recent years a rare deployer of balloons, has sent more than 1,000 south - most laden with garbage and some with what appeared to be animal faeces.
That's ratcheted up tensions between the countries which technically remain at war after the 1950-1953 Korean War ended in an armistice agreement and not a peace treaty. South Korea on Sunday resumed loudspeaker broadcasts directed at the North for the first time since 2018.
How effective the balloons are is a matter of debate, with no independent verification possible of where they land or what average North Koreans might think about the contents.
A second member of the group said he was encouraged by Pyongyang's anger over balloons from South Korea, saying it shows that activists' balloons and their payloads are having an effect.
The group's members declined to be identified, worried about harassment from South Koreans critical of such activists, a potential crackdown by South Korean authorities or reprisals by North Korean agents.
THE PAYLOADS
Filled with hydrogen, the group's smart balloons can carry payloads of up to 7.5 kilograms.
In a small rented apartment in Seoul, the team uses 3D printers to build white plastic boxes and some connective parts. Wires, circuit boards, and timers bought from Chinese and South Korean e-commerce websites are used to make devices that control the dispersal of the balloons' contents.
Most balloons contain devices pre-programmed to scatter 1,500 leaflets, 25 at a time, taking into account the hoped-for flight path, wind and other weather conditions.
This year, some balloons are carrying speakers attached to small parachutes that blare pre-recorded messages critical of North Korean leader Kim Jong Un.
A typical cargo might be six speakers and six other bundles, each containing a bible and a short-wave radio, according to the first member of the group, who defected from the North in the 1990s and is in charge of technical development.
The lantern-shaped speaker devices are constructed using a waterproof box, lithium-ion batteries and an amplifier. When deployed, three small rainbow-coloured parachutes on top of the speaker open while a foam base helps absorb any landing shock.
They then blare 15 minutes of North Korean songs and messages recorded in a North Korean accent and pause for 30 minutes before beginning again. The batteries can last for 5 days.
"Get rid of the Workers' Party, then Joson can survive. Kim Jong Un is a traitor that opposes unification," part of the recording says.
Another key technical advancement made over the past two years has been altimeter-linked valves that automatically prevent the balloons from going too high, making for a more stable flight, though the balloons are still at the mercy of the weather and their flight paths cannot be controlled.
The group estimates its balloons have a 50-60% success rate of going further than a few dozen kilometres north of the border. That's better than older-style balloons that often don't go that far, can quickly go off course and are only able to drop one parcel of leaflets.
PUSHBACK
A handful of groups in the South regularly send balloons to the North, activists estimate.
The South Korean government once sent its own leaflets over but abandoned the practice more than a decade ago. It instituted a ban in 2020 on national security grounds. But when a court struck down that ban last September saying it violated the constitutional right to free speech, groups ramped up balloon flights from the South.
South Korea's Unification Ministry said it respects the court's decision. It will take appropriate measures if necessary, it added, without elaborating.
North Korean officials have called South Korean leaflet activists "human scum" and in 2020 demolished an inter-Korean liaison office during a spat over leaflets. In 2022 they claimed that these "alien things" could carry the coronavirus.
The flights are also controversial in South Korea where some residents have clashed with activist groups, arguing the balloons are confrontational and put them at risk.
The smart balloon group said South Korean marines near the border have previously verbally warned them away from conducting launches. The military has said troops have no right to restrict balloon launches by private groups.
