 Major restoration and foundation work began in the 1990s to stabilize the building . Engineers excavated under the cathedral between 1993 and 1998 . They dug shafts under the cathedral and placed shafts of concrete into the soft ground to give the edifice a more solid base to rest on . These efforts have not stopped the sinking of the complex , but they have corrected the tilting towers and ensured that the cathedral will sink uniformly . 
 Major restoration and foundation work began in the 1990s to stabilize the building . Engineers excavated under the cathedral between 1993 and 1998 . They dug shafts under the cathedral and placed shafts of concrete into the soft ground to give the edifice a more solid base to rest on . These efforts <a href='been_done' title='1'><mark>have</mark></a> not stopped <a href='None' title='2'><mark>the</mark></a><a href='negative_concord' title='3'><mark>sinking</mark></a> of the complex , but they <a href='been_done' title='4'><mark>have</mark></a> corrected the tilting towers and ensured that the cathedral will sink uniformly . 
Major restoration and foundation work began in the 1990s to stabilize the building . Engineers excavated under da cathedral between 1993 and 1998 . They dug shafts under the cathedral and placed shafts of concrete into tdasoft ground to give thdadifice a more solid base to rest on . These efforts done not stopped no sinking of the complex , but they done corrected the tilting towers and ensured that thedathedral gon sink uniformly .