Turbidity currents are large sediment-carrying submarine avalanches that flow over the continental slope and through submarine channels to the deeps of the world's oceans. For more than 50 years researchers have extended the hydrostatic pressure simplification, which is valid in most hydraulic analyses of flow in rivers and channels, to analyses of turbidity currents. In this paper we present the first measurements of fluid pressure beneath experimental turbidity currents. Basal fluid pressures are much lower (30%–45%) than those predicted from bulk flow density, in proportion to the kinematic energy density of the flow, a parameter usually referred to as the dynamic pressure. This invalidates the commonplace assumption of a hydrostatic pressure regime in turbidity currents, and necessitates the incorporation of dynamic fluid pressure as a physical parameter in the analysis of turbidity currents. The contrast between turbidity currents and free surface flows is caused by the difference in their density structure. We highlight current enigmas and gaps in our knowledge of submarine currents, for which our result opens new opportunities of investigation.