 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 . 
