Deep drilling of borewells to hundreds of meters – depth is increasingly frequent for water exploration in fractured crystalline rocks. At the same time, many studies provide evidence that hard-rocks areas are characterized by a shallow higher-permeability zone (‘‘active'' zone) that overlies a deeper lower-permeability zone hosting little flow (‘‘inactive'' zone). Consequently, the yields of borewells decrease dramatically with depth. This gap between hydrogeological practice and science can be explained by a well-known behaviour in psychological sciences. The escalation of commitment consists in justifying increased investment in a decision, based on the cumulative prior investment, despite new evidence suggesting that the decision was probably wrong. This behaviour leads to the sunk cost fallacy of deep drilling: drilling a borehole to unreasonable depth in the hope of recovering the money wasted to drill the first dry meters. As water experts, we should contribute to end this irrational trend of wells deepening in hard-rocks.