Understanding the spatial nature of the world is necessary in everyday life. Not only do people move about in 3D space, but spatial thinking is also important in many science, technology, engineering, and mathematics (STEM) disciplines. This may be especially true in the geosciences, where learners often encounter unique spatial reasoning challenges that are central to the discipline. The authors, who are educators, developed a lesson, adapted from a common college-level introductory geoscience laboratory activity, to improve understanding of Earth's subsurface features and to facilitate penetrative thinking, where one visualizes spatial relations inside an object. The project-based learning (PBL) lesson poses the question "What does the Earth's subsurface look like?" and focuses on three skills: (1) visualizing three dimensions from two-dimensional representations; (2) penetrative thinking; and (3) locating self and other objects.