When children are out of their teacher’s sight, they often initiate play spontaneously. If this occurs at an unpermitted time or in an unpermitted way, then such play is called "underground play". This study aims to understand how young children initiate underground play and solve conflicts during underground play. A qualitative study design was employed, and participant observations were taken for data collection. A preschool indoor classroom with three separated spaces beneficial to underground play was purposively selected for this study. There were 28 young children aged 3-6 in the mixed age class. This study collected 128 observations which the researchers entered the class three or four times a week, and each time began at 8:00 and ended at 16:00. Additionally, interviews with the teachers were also collected for member checking. This study adopted the method of constant comparative analysis for data analysis, including unitizing, categorizing, framing, and checking. Through data analysis, this study found that the young children applied mimicking others, verbal challenge, and physical triggering to initiate underground play. When conflicts occur, the children applied the teacher’s role and hidden physical attacks to attain control, implying the manipulation of social power by young children at school. During underground play, children were also engaged in their social grouping. Finally, this study provides suggestions for early childhood education and future studies.