The narrow regions of intense vertical mixing to great depths in the ocean are discussed, with emphasis on such a region or “chimney” observed recently in the Weddell gyre. It is deduced that such chimneys are the result of surface wintertime cooling. Application of two models of the resulting vertical convection shows that the entire of the area of the Weddell gyre is prone to overturning; yet only one narrow region apparently did so. This shows that a preconditioning process is responsible, which preselects a narrow area and reduces its vertical stability. Baroclinic instability of the mean flow is capable of producing cyclonic and anticyclonic eddies with horizontal length scales of the same width as the observed chimneys. At the center of the cyclonic eddies, the vertical stratification is greatly reduced in the top 300 m, thus acting as an efficient preselection mechanism, at the onset of winter cooling. Such a theory can also explain the appearance of bottom water in the Greenland Sea, where no examples of vertical homogeneity have ever been observed. Estimates of the number of chimneys necessary to form the bottom water make it likely (82%) that no chimney would have been observed to date.