Sometimes history is just plain silly.
In 1393 Gian Galeazzo Visconti - the most powerful of all the Viscontis - wanted just another city. In the years before he had conquered Verona, Viczena and Pavia, and now he was out to besiege Mantua on his way to a Kingdom of Northern Italy.
To add Mantua to his collection, he shelled out the gigantic sum of 300.000 golden florins to build an equally gigantic dam near the small city of Valeggio that would cut off Mantua from the river Mincio - a river that flows out of the Lake of Garda on its southern end.
In only two years the 600 meters long, 10 meter high and 26 meters wide dam was built - but, alas, never used for its original intention. The water of the Mincio continued to flow merrily towards Mantua, never to be disturbed by the dam.
After Viscontis death in 1402 nobody cared for it anymore, and its rather slow detoriation begun. But since this monument of senselessness is quite resilient, this process of detorioation is far from being finished today. The dam is still in use today as a bridge, and cars and busses pass it frequently. Do all passing it really know just how absurd this dam is?
Ponte Visconteo, seen from the Castle of Valeggio
An addendum: The dam was large enough to cause another unbelievable story. It forced the Venetians - at war with Milan in 1437 - to hoist a complete fleet of ships over the northern mountains of the Lake of Garda. Six galleys, two galleons and 26 barques were moved within two years over those quite high mountains, and on November 20th 1439 the battle with the Milan fleet began. The Venetians lost, and this time had the brilliant idea to built new ships directly at the lake. With those they finally defeated the Milans in April 1440.