After isostatic correction for their sedimentary fill, the depths of two small oceanic basins in the southern Scotia Sea suggest that both started to open in mid to late Eocene times. Models of the short magnetic anomaly profiles across the basins provide support for these determinations. The location of the basins, adjacent to the present-day Drake Passage marine gateway, and the timing of continental stretching leading up to their opening, during the Ypresian (early Eocene) onset of global cooling, mean that their importance is potentially far greater than their small size implies. Extension in the region of the two basins would have opened Drake Passage to shallow or intermediate depth oceanic circulation between the Pacific and Atlantic oceans for the first time. This coincided with a reorganisation of vertical mixing patterns in the global ocean, a shift in the site of carbon sequestration from coal swamps and peatlands to ocean sediments, and the onset of a long decline in atmospheric carbon dioxide concentrations. Cenozoic global cooling may therefore have begun as a result of the shallow opening of Drake Passage.