Scientists, and science in general, move from the unknown to increasing levels of certainty. Teaching students about science means encouraging them to embrace and investigate the unknown, make reliable scientific claims, justify those claims with evidence, and evaluate the quality of the evidence. In all areas of science--and especially in frontier science, in which claims can be disputed and changes arise with the discovery of new evidence--this level of critical thinking is key. Schools often teach "known" science. By incorporating the unknown into the curriculum, schools can engage students in scientific ways of thinking. This article describes The Concord Consortium's "High-Adventure Science" project. The project's goal is to bring frontier science into the classroom, allowing students to explore questions in Earth and space science that scientists are currently investigating. (Contains 6 figures and 1 online resource.)