High school mathematics needs to reflect on the learning process, the process of understanding mathematical problems from real-life phenomena and solving them mathematically. Therefore, there is a need for teaching materials, including mathematical activities that move back and forth between real phenomena and mathematical abstraction. In our most recent study, we examined mathematical activities in engineering and technology and presented some teaching materials for the new subject “Inquiry-Based Study of Science and Mathematics” and STEM. In this paper, we studied salt mountains as the teaching materials of mathematical activities and performed the teaching practice using the mathematical activity “Salt Mountains” for high school students. In addition, we will clarify the characteristics and educational value of the teaching material through practice, analyze the ridgeline of some kinds of salt mountains that have never been elucidated by previous studies, and introduce mathematical activities involving numerical calculation.