In the balloon warfare between North Korea and South Korean activists, one Seoul-based group has honed its tech expertise to develop balloons capable of dispersing leaflets and electronic speakers hundreds of kilometers across the border.

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 favorable winds blow north, the secretive group flies the balloons — mostly under the cover of darkness.