Why doesn’t the Leaning Tower of Pisa fall over?

In 1990, the Italian government asked smart engineers to fix the Leaning Tower of Pisa. This tower had been leaning for 800 years, and previous attempts to fix it didn't work. The engineers used computers to figure out that the tower was about to fall if it leaned more than 5.44 degrees. Right now, it was already leaning at 5.5 degrees. They didn't know how the tower was still standing, but they needed to solve this problem quickly.

To understand why the tower leaned, we have to go back to the 12th century. The people in the city of Pisa wanted to make their cathedral square look amazing. They built a big church and a huge baptistry. Then, in 1173, they started building a separate bell tower, the Leaning Tower of Pisa.

The builders were really good at what they did, but they didn't know much about the ground underneath. Pisa was named after a Greek word that means "marshy land," and that's exactly what was under the city – soft clay, mud, and wet sand. The ancient Romans had dealt with similar problems by using big stone pillars called piles that went deep into solid ground. But the tower's architects thought a three-meter foundation would be enough for their tower. That was a big mistake.

In less than five years, the tower's southern side was already sinking into the ground. Normally, this would have been a big problem, but construction stopped for almost 100 years because of wars. During this time, the ground settled a bit. When they started building again, they made the tower taller on one side to fix the tilt. But this made that side sink even more.

For hundreds of years, engineers tried different things to fix the lean. They dug around the base in 1838, but it made things worse. In 1935, they put mortar in the foundation, but it didn't spread evenly, and the tower sank again. All these failed attempts and the sinking ground brought the tower closer to falling over. But because they didn't know the exact nature of the ground, they couldn't be sure how much more it could lean before falling.

After World War II, researchers developed tests to understand the ground better. In the 1970s, engineers used new technology to figure out the tower's center of gravity and how stiff the ground was. With this information, they could predict how much they needed to dig under the tower and where to put support.

In 1992, the engineers dug tunnels under the tower's north end to remove some soil and then added heavy lead weights to balance it temporarily. They also used steel cables to anchor the base. After more than 600 years since it was built, they finally straightened the tower, but not completely. It still leans a bit, but it's safe now.

No one wanted the tower to fall, but they also didn't want to lose its famous lean. Today, the tower stands at about 55 or 56 meters tall, and it should stay that way for at least 300 years. It's a symbol of imperfection that people can admire.

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