Mixing of water masses from the deep ocean to the layers above can be estimated from considerations of continuity in the global ocean overturning circulation. But averaged over ocean basins, diffusivity has been observed to be too small to account for the global upward flux of water, and high mixing intensities have only been found in the restricted areas close to sills and narrow gaps. Here we present observations from the Scotia Sea, a deep ocean basin between the Antarctic peninsula and the tip of South America, showing a high intensity of mixing that is unprecedented over such a large area. Using a budget calculation over the whole basin, we find a diffusivity of (39 +/_ 10) x 104 m2 s-1, averaged over an area of 7 x 105 km2. The Scotia Sea is a basin with a rough topography, situated just east of the Drake passage where the strong flow of the Antarctic Circumpolar Current is constricted in width. The high basin-wide mixing intensity in this area of the Southern Ocean may help resolve the question of where the abyssal water masses are mixed towards the surface.