The world's population is growing by about 80 million people a year, implying an estimated increased freshwater demand of about 64 billion cubic meters annually (World Water Assessment Programme, 2009, Water in a Changing World: United Nations World Water Development Report 3, Chap. 1, p. 3-21). Groundwater depletion, which reduces the amount of valuable water available for drinking and food production, has become a global crisis. Decision-makers at all levels desperately need to understand the unseen system beneath their feet and its connection to the earth's hydrologic cycle. Yet teaching groundwater concepts is extremely challenging; foundational misconceptions about groundwater's location, movement, and connection to the hydrologic cycle are common. Quality, multimodal instruction, and assessment of groundwater topics will help to clarify elementary students' misconceptions and assist them in constructing accurate mental models of the groundwater system. This study examines student responses to different forms of assessment, including drawing prompts, to determine the best way to ascertain what students really know about the groundwater system, a key component of the larger water cycle system. The assessment tools included dichotomous, multiple-choice, and drawing questions used to elucidate students' conceptualization and understanding of the groundwater system. Assessment results show that students who are able to answer objective questions about groundwater are not necessarily able to demonstrate their knowledge; calling into question their conceptual understanding of the system.