Why is Venice under water?

The city of Venice, Italy, is not actually under water, but on water instead. The city was built on a collection of small islands starting in the Fifth Century.

Most of the buildings of the city are erected on wooden piles that are very closely spaced. Many of the piles are made from trunks of alder trees, a type of tree that is known for its resistance to decay when submerged by water. The city of Venice has always been threatened by flood tides between autumn and spring. Nowadays, engineering work is constantly completed to make sure the floods do not affect the city.