Ozone miniholes appear on total ozone maps as localized ozone minima with horizontal extents of a few hundreds of kilometres. They are characterized by a rapid and small- scale appearance of a columnar ozone decrease with an equally rapid recovery after a few days. They are frequently observed at Northern Hemisphere mid-latitudes in winter. Evolving too rapidly to be the result of an ozone chemical destruction, miniholes should be the result of meteorological processes. According to some authors, miniholes should be due to the northeast motions of air patches with low total ozone content. However, several studies attribute the formation of ozone miniholes to the uplift of air masses, which decreases the ozone columnar content by simply decreasing the pressure thickness of the ozone layer, without changing the mixing ratio. According to these studies, the latter mechanism explains the main reduction of ozone that occurs between the tropopause and the ozone maximum during an ozone minihole event. A region of extreme low ozone values passed over Europe from 27 to 30 November 2000. The total ozone values were measured with the Total Ozone Mapping Spectrometer (TOMS). A radio sounding, launched on 29 November 2000 from Payerne at the place and time of the deepening of the minihole, allows us to perform a detailed analysis of its formation mechanism. It is shown that the uplift of isentropic surfaces plays an important role in the columnar ozone decrease and explains the lower part of the depleted ozone profile. However, the deepening of the minihole is explained by another mechanism: namely, at this time the minihole air column intersects the polar vortex at high altitudes and then encounters ozone-poor air masses.