Torrential rains and unusually high tide has caused widespread flooding in Venice, Italy, leaving 75 percent of the city under water this week.

Although high tide, known as Acqua Alta, is common during this time of year, Sunday's water levels reached 5 feet, the sixth highest since records began in 1872.

Many Venetians and tourists took the opportunity to throw on bathing suits and enjoy a swim in the city's flooded streets and squares — though we're not sure how good of an idea this is.

The city does not have a complete modern sewage system, meaning its canals are also its sewer system. This poses some obvious health and sanitation concerns.

An Italian hotelier, whose hotel entrance was flooded, described the health risks to the Sydney Morning Herald. He said: "this is not clean water — you need to mop with disinfectant twice after it goes down."

A video called Venice Backstage describes how the floating city works, including its old sewer system. We've highlighted the main points, but recommend checking out the full documentary to learn more about the city's canals, buildings and history.