Big Ben

Because Big Ben's foundations rest on a layer of gravel with London clay underneath, the tower began to lean slightly toward the northwest.

An excerpt from the article 22 facts about Big Ben

In the 1990s, thousands of tons of concrete were pumped into the ground beneath the tower to stabilize it during the construction of the Westminster section of the tube line (Jubilee line). Experts believe that the current tilt of the tower will not be a problem for the next 4000 to 10.000 years.