What was the issue with the Leaning Tower of Pisa?

The Leaning Tower of Pisa is the Prime example of why you do not build castles on sand. 

The entire area was once marshy swampland.   The soil was sandy, at best. 

and once work started......the enormous weight of the solid MARBLE monument,  took it's toll. 

In fact, work on the tower was started and stopped several times,  over a period of 130  years ! Wars and lack of funds, and other reasons, caused work to come to a halt.   

As the unfinished base sat undone for decades, it started sinking into the sandy soil.   When work resumed,  they wanted to correct the now obvious tilt, so they made the walls a little higher on the other side to balance it out. 

The extra weight, then caused the tower to become straight again,  but as worked stopped a second time.........yup........started leaning the other way now, because of that extra, uneven weight.  

When work resumed again years later.......they tried the same trick..... giving the other side now the higher walls.......

giving the whole thing a smashed wedding cake look.  Realizing this just wasn't working out,  they called it a day,  threw some bells at the top.....which only added more weight......and caused a significant lean. 

Various tricks over the years  have tried to straighten it back up, some failing and making it worse.........causing real fear the whole thing would eventually topple over..........but the last effort was more advanced, using modern technology and they were able to bring it back  a few inches. 

It's the same old story... You know how Italy works? The soils choosen where to build the tower was wrong, it was too marshy, but the engineer told "I don't sucho" and the tower had been built the same way.

I'm Italian, I appeal to you tourists, Please come to my country and hold up the tower, it could fall down If you don't help us to fix it.

Unstable soils underneath it. It's the lean that made it famous, so engineers got in there and stabilised the base as best they could so it still leans but is less likely to fall over soon. 

And FYI:

http://slowitaly.yourguidetoitaly.com/2014/10/lean...

It was built on  unstable subsurface soils. The weight of the building caused it to sink more on one side than the other. 

rick is right ..poor sub soil ..nothing wrong with foundations except one section n was built on silt ..its very strange when you see it for first time ..first time i went up it ..over 50 years ago i cant remember safety barriers!..

the foundation sunk on one side, it wasn't built on bedrock.