Craig Simmons’ work on variable-density flow phenomena is internationally recognised as revolutionising understanding of groundwater and groundwater processes. He drove development of new mathematical models capable of greater insight and accuracy by addressing the limitations of classical fluid mechanics in groundwater geoscience. Simmons made the first direct observation of gravitational instabilities in a natural groundwater setting, confirming his theoretical predictions about the nature, complexity, and significance of unstable groundwater phenomena. His science underpins a new knowledge framework, establishing a new modelling paradigm to inform environmental management and policy issues as diverse as food and water security, coal seam gas, fracking, nuclear waste disposal, mining and energy.