ABSTRACT The stabilisation of the Tower of Pisa has been a very difficult challenge for geotechnical engineering. The Tower is founded on weak, highly compressible soils and its inclination has been increasing inexorably over the years to the point at which it was about to reach leaning instability. Any disturbance to the ground beneath the south side of the foundation is very dangerous ; therefore the use of conventional geotechnical processes at the south side, such as underpinning, grouting etd., involved unacceptable risk. The internationally accepted conventions for the conservation and preservation of valuable historic buildings, of which the Pisa Tower is one of the best known and most treasured, require that their essential character should be preserved, with their history, craftsmanship and enigmas. Thus any intrusive intervention on the Tower had to be kept to an absolute minimum and permanent stabilisation schemes involving propping or visible support were unacceptable and in any case could have triggered the collapse of the fragile masonry. In 1990 the Italian Government appointed an International Committee for the safeguard and stabilisation of the Tower. It was conceived as a multidisciplinary body, whose members were experts in arts, restoration and materials, structural engineers and geotechnical engineers. After a careful consideration of a number of possible approaches, the Committee adopted a controlled removal of small volumes of soil from beneath the north side of the Tower foundation (underexcavation). This technique provided an ultra soft method of increasing the stability of the Tower, which is completely consistent with the requirement of architectural conservation. The paper reports the analyses and experimental investigations carried out to explore the applicability of the procedure for the stabilisation of the Leaning Tower of Pisa. All the results having been satisfactory, the actual underexcavation of the monument was carried out in the years 1999-2001 ; the results obtained are presented and discussed.