The ocean is stably stratified, mainly because the temperature decreases downwards. The overturning cirulation that connects the surface and the deep ocean must therefore penetrate through surfaces of constant density, which is made possible by small-scale mixing. Where do this mixing and the resulting upwelling and downwelling occur? How much energy is required for the mixing, and what is the energy source? These questions are fundamental for oceanography, and crucial for understanding past climates and predicting the future climate. I will describe how the views on these questions have evolved over the past decades.