This thesis explores the dynamics of explosive volcanic eruption columns in the deep-ocean. Motivated by the observations of pyroclastic deposits on the sea floor at depths of up to 4 km, we investigate the initial mixing of erupted hot fragmented magma with the ambient seawater and explore the density and spatial evolution of the subsequent flow that forms. In chapter 2, we consider the initial mixing and rise of an explosive eruption in a quiescent ambient environment via a series of numerical integral models and find that a submarine eruption column tends to evolve as a turbulent particle-laden fountain. We investigate the controls on the density evolution of the flow, such as particle separation, and consider the particle dispersal mechanisms of these complex multi-phase flows. In chapter 3 we explore the dynamics of particle-laden fountains in a stratified environment, and identify two distinct flow regimes as a function of the average size of the particles, in which the flow is controlled by the ambient stratification or by the separation of particles. We build two numerical models for the rise and fall of particle-laden fountains in a linear density stratification. We contextualise the findings of this study for submarine explosive eruptions and estimate the dispersal distances of pyroclasts in the submarine environment. Next, in chapter 4, we consider the effect of a uniform lateral crossflow on the dynamics of particle-laden fountains. We perform a series of analogue experiments and examine the impact of particle separation on the structure of the flow. We develop the theory for single-phase fountains in a crossflow to identify the location of particle separation on the flow. Using field data on the dispersal of pyroclasts from deep-submarine eruptions, we show how the present work can be used to constrain eruption parameters, such as mass eruption rate. In chapter 5, we depart from the study of submarine volcanoes and consider a complimentary problem, motivated by the sediment plumes formed during the operations of deep-sea mining. We explore the dynamics of dense sediment plumes in a uniform crossflow and consider the evolution of the gravity currents that form when these plumes interact with a solid boundary. We investigate the effect of the crossflow speed on the morphology of the gravity currents formed and compare the propagation of the flows with the classical theory of turbulent gravity currents. Finally in chapter 6, we summarise the work of this thesis and build a framework to help interpret the dynamics and deposits of submarine eruptions using the analysis presented throughout.