Common ancestor of Han Chinese, Japanese and Koreans dated to 3000 – 3600 years ago…
The Leaning Tower of Pisa is a cathedral bell tower in the Italian city of Pisa. It began construction in 1173 AD and the foundations were built on sand and silt. By the time the third floor was built, the tower was already leaning.
Engineers tried to correct the tilt by building the rest of the floors with the height of the floor in the inclined direction greater than its height in the other direction. However, this only made the tilt worse.
The Leaning Tower of Pisa did not collapse for several reasons. First, the construction took 199 years, which allowed the soil to compact and reduce the rate of tilt. Second, the clay soil helped to keep the tower from collapsing. Third, the engineers calculated the center of gravity of the tower and knew that it would not collapse until the tilt reached 5.44 degrees.
In 1990, the tower was closed and engineers began work to stop the tilt. They dug deep holes in the ground and installed iron cables through the holes. They also pumped liquid nitrogen into the soil, which caused the water to freeze and expand. This caused the soil to subside and the foundations to tilt in the opposite direction.
In 1999, the tower was reopened to the public. The tilt had been reduced to 4 degrees, which is what it was at the beginning of construction. The engineers could have made the tower vertical, but they did not want to lose its fame and tourist value.
It is now estimated that the tower will not collapse for at least 300 years.