Standing 186 feet tall, the Leaning Tower of Pisa is one of the most famous buildings in the world. What caused it to start tilting?

The Leaning Tower of Pisa wasn't meant to lean. When construction began in 1173, the builders were up to the third story by the time they realized the ground was too soft. The foundation was destabilized by this, resulting in its now nearly 4-degree lean. A 5.44-degree lean will topple the tower completely, according to physicists.

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