This study was designed to explore the learning mechanism and its treasures within an undocumented migrant workers' voluntary community in Korea. Based on a qualitative case study, major findings suggest that various modes of learning mechanisms acted as crucial foundations of the community's diverse learning paths and participants' social engagement. Four mechanisms steered the community towards a sustainable learning environment: constructiveness, communicativeness, collective autonomy and practicality. Through these, unauthorized migrants were able to have their "voices" heard in the wider society. These learning treasures have implications for both theory and practice, and for empowering underprivileged groups. They could also widen the horizon of understanding human learning in an informal learning context.