Pisa Tower

I visited once when I was a kid and traveling in Italy with my parents. I know that it's almost 1000 years old and that it took about 100 years to be finished because construction was stopped a few times. ALthough some people commented that maybe it was designed to lean from the beginning apparently that was not the idea, it was supposed to be straight, but it started leaning to one side during construction. It seems to have something to do with the soil where it was erected.