Tower of pisa
The tower leans because it was constructed on fine sand, clay and shells, all of which are bad base materials. Many thngs have been done to try to correct it but little have worked. These attempts include wrapping steel rings around base of tower, adding a concrete base to support it, adding counter weights and steel cables but none worked. In 2008 they took out over 70 metric tons of soil and this finally stopped the tilting, however it is stuck at and angle of 4 degrees south, but the attempt did some more good, it was leaning at 5.5 degrees. But in the end, all f these were used n the final plan, the plan. which halted it.
First they wrapped steel plates round the bottom 2 floors so the stone did not fracture, if it did then the tower would fall. Then theyput a counter weight of 750 metric tons on the north side so the centre of mass was in the middle. Next they poured a concrete ring around the base of the tower which was connected to cables deep under ground. Finally they removed dirt from under the dower with 20cm drills so the surface would Even out. The tower is now stuck facing 4 degrees north. However it is not in any danger of falling because they have supported it enough for the next 200 years, in which time humanity will have blown its self up or have adequate technology to fix it again.