The foundation was built on less solid ground than it should have been, and was not built well enough to hold up the whole tower. It started out straight, but started leaning shortly after it was built. I believe they've done some work on it to stabilize the ground under it, however, by that time it was so famous that they didn't want to put it back up. Nobody wants to go see the straight-up-and-down tower of Pisa.

What was the foundations of the tower?

Was it weak soil or sandy foundations? I need to find out.

It started tipping in construction.

THE LEANING TOWER OF PISA WAS SUPPOSE TO BE STRAIT UP BUT THE PEOPLE THAT WAS BUILD THE LEANING TOWER OF PISA WERE BAD BUILDERS THEY MAD THE BUILDING TILTING TO THE SIDE

BECAUSE IT'S MADED IN SOFT SOIL, SO AT MAKING 3TH FLOOR, THE BUILDING IS LEANING TO SOUTH, UNTIL FINISH TO THE NORTH

It wasn't built on good foundations and good bedrock. They had barely got three 'layers of the cake' up before it started leaning one way. So they tried to correct it by leaning the new levels slightly away from the original incline. Then it started leaning in the other direction so again they built away from the lean. When you actually look at it or go up it, it is not a straight tower leaning in one direction, it is a distinctly wonky tower than leans in one direction up to the third floor, then is built at a slightly different angle, then higher up at a different angle again. It is very funny to look at. It was getting to the point where the angle of the lean was dangerous, so a bit over 10 years ago they invested millions in using weights amongst other things to correct the lean back to what it was a couple of hundred years ago. Obviously they don't wan't to correct it too much - think of all the money they make off the tourists (about 12-15 euros to go up it when I went up) It gives you a very funny feeling going up the spiral staircase inside it because the staircase is drunken too. First it leans to the left, then it gets very steep, then to the right and then you are tipped forward a bit. Not good if you are scared of heights :-)