A theoretical model of metacognition in complex modeling activities has been developed based on existing frameworks, by synthesizing the reconceptualization of metacognition at multiple levels by looking at the three sources that trigger metacognition. Using the theoretical model as a framework, this multiple-case study explores students' spontaneous metacognitive activities while they collaboratively solve complex mathematical modeling tasks. This study used a series of model-eliciting activities--a type of problem-solving activity in which participants are required to verbalize their thoughts while working within a group--as an authentic method for analyzing verbal metacognitive actions. This study identified the circumstances facilitating or interfering with students' spontaneous metacognitive activities. The findings of the study enrich our understanding of how to design metacognitive learning environments. The current study has the potential to guide teachers, teacher educators, and curriculum developers to create circumstances that support students' spontaneous development of metacognitive abilities. It also has the potential to guide the development of effective instructional methods to integrate these circumstances into existing curricula.