The tower's tilt began during construction, caused by an inadequate foundation on ground too soft on one side to properly support the structure's weight. The tilt increased in the decades before the structure was completed, and gradually increased until the structure was stabilized (and the tilt partially corrected) by efforts in the late 20th and early 21st centuries.
You can find more information of Leaning Tower of Pisa on Wikipedia. And here, a small selection of analog photos of this famous Pisa tower.
|the leaning tower of pisa by bevtales|
|Pisa by shiunkle|
|Torre pendente di Pisa by tedlum|
|Very wonky by lissyloola|
|photo of the day 8/8/09 - The Leaning Tower of Pisa by Pete Repka|
|The Cathedral, Pisa, Italy by Blue Pelican|
|Slow motion by Bjørn Giesenbauer|
|Piazza dei Miracoli - Infrared by FranzPisa|
|Piazza dei Miracoli by Gabriele Cappello|
|untitled by vic xia|
|Pisa by Jade M. Sheldon|
|Frank Ocean, sweet life by girl-from-ipanema|
|Pisa by philosli|
|pisa two by kadmon|