They used to say "see Venice and die". The impossible city-on-the-sea has long been a bucket list staple for travellers lured by its water and its stone.

But now Venetians are questioning whether the city itself will survive long enough for coming generations to experience.

A masked reveller poses during the Carnival. ( Reuters: Manuel Silvestri )

Venice is facing multiple threats but, as Foreign Correspondent discovers, the sheer weight of visitors is the one that many locals find the most overwhelming.

Just near the famous Rialto Bridge spanning the Grand Canal, a clock tracks the local population decline.

It now registers just under 53,000 locals — less than half the number 60 years ago.

At this rate, Venice will be all but empty of Venetians in about 50 years.

Meanwhile, 25 to 30 million tourists will arrive this year.

The 'eat-and-run' visitors

It's a bitterly cold February morning in Venice but with each passing minute, getting around the city is becoming more difficult.

Preparations are underway for the annual pre-Lent Carnevale festival and the already-crowded laneways are filling with more and more tourists.

Gondolas navigate through a narrow passage in Venice. ( Foreign Correspondent: Tim Stevens )

Visitors from China are by far the most visible group wandering the city's tourist hotspots.

They're often pilloried by locals as the main perpetrators of what's called "eat-and-run" tourism.

The eat-and-run visitors come on tours with tightly choreographed schedules.

They congregate in high-traffic areas, eat in designated restaurants and tend to buy from stalls selling mass-produced souvenirs.

Many eat-and-run tourists find their way to St Mark's square, one of Venice's busiest attractions. ( Reuters: Manuel Silvestri )

The arrival of the eat-and-run coaches is a sight in itself. They come in convoys, bus after bus, doors folding open and tour guides emerging, flag in hand, with dozens of clients in tow.

The visitors are clearly thrilled to be on Italian soil, if only for a fleeting moment.

A bus comes to a halt and disgorges another load of visitors, eager to make the most of their time in the city.

But this group hardly even qualify as day trippers — one tourist says they will squeeze in a gondola ride and spend just three hours in the city.

Visitors crowd along the Cannaregio Canal to watch a water parade marking the beginning of carnival season. ( Reuters: Manuel Silvestri )

Such a hasty experience of Venice is hard to comprehend for the locals.

They urge visitors to take the time to get lost in the city's myriad alleyways and hidden squares, or take a stroll over the beautiful bridges that crisscross the canals.

Venetians also accuse eat-and-run visitors of spending very little in the local economy, while swelling the crowds and putting pressure on infrastructure.

Is it fair to blame Chinese tourists?

Last year alone there was a 20 per cent increase in Chinese tourists in the Venice area, according to Venice-based academic and marketing expert, Victoria Ying Liu.

But it's not just Chinese visitors who are swelling the city's numbers.

Traditional artisans like mask maker Marilisa Dal Cason struggle to compete against cheap tourist souvenirs. ( Foreign Correspondent: Tim Stevens )

"We have more German people, people from the UK, people from France, Spain," Ms Ying Liu says.

"So actually they are much more than Chinese people because it's only 10 per cent of Chinese who can travel."

That's because only about 10 per cent of China's 1.4 billion citizens currently have a passport. That's set to change, with potentially major ramifications for not only Venice, but tourist numbers worldwide.

"The potential of the Chinese market, I think, is huge," Ms Ying Liu says.

Gondoliers row as tourists enjoy a ride along a canal. ( Reuters: Manuel Silvestri )

She predicts eat-and-run visitors will decline and future Chinese visitors will stay longer and bring more money into the city.

"Chinese people, they like shopping — they love shopping — they spend a lot when they are travelling," she says.

"So I think it's a resource for the city."

Airbnb killing the city

If anything it's the conventional, longer-staying tourists who are having the most severe impact on residents.

Locals are being squeezed out of their homes as landlords and agencies like Airbnb convert residential apartments to lucrative tourism rentals.

There are nearly 8,000 Airbnbs in Venice's small city centre and no restrictions on the company's growth.

Long-term renters, often middle class, say they are being priced out of the market. Rents swallow up about half of their income on average.

Housing activist Nicola Ussardi says Venice residents are being forgotten as tourists swamp the city. ( Foreign Correspondent: Tim Stevens )

"The battle against Airbnb is very difficult," says Nicola Ussardi, co-founder of Assemblea Sociale per la Casa (ASC), a community housing group dedicated to helping Venetians find homes.

"It's like trying to win against Coca Cola."

It's not that the Venetians want tourists to stop coming — visitors inject about $9 billion a year into the local economy — but they desperately want more say over the seemingly uncontrollable influx of people.

And it's a mighty struggle.

Many find the massive cruise ships that sail right into the Venice lagoon during the six-month tourist season intolerable.

"Cruise ships are not giving money to the city": Venice hosted 594 cruise ships last year. ( Reuters: Stefano Rellandini )

Last year the city hosted 594 cruise ships. They tower above the city skyline and critics say the currents created as the ships pass by are damaging the Renaissance buildings.

There is some dispute about the extent of erosion, but the damage from pollution is widely accepted.

One cruise ship has been shown to emit as many air pollutants as 1 million cars daily.

No Grandi Navi posters are a part of the anti-cruise ship movement in Venice. ( Foreign Correspondent: Tim Stevens )

"They are destroying Venice, they are physically destroying Venice, physically destroying our lungs," activist Tommaso Cacciari says.

"It's kind of the most invasive, stupid tourism I have ever seen."

Mr Cacciari is a founding member of the anti-cruise ship movement "No Grandi Navi", or No Big Ships.

Each year the group sails a noisy flotilla of small boats around the ships to protest against their presence. But nothing seems to change.

"We are destroying the city. We are choosing to kill it for money," Tommaso Cacciari says. ( Foreign Correspondent: Tim Stevens )

"Cruise ships are not giving money to the city, are not giving money to the workers, the bars and to the taxi drivers," Mr Cacciari says.

"They're keeping the money concentrated in a very few hands, and these few hands are very powerful. Some politicians are very tied with these interests."

In the long run though, crowds of tourists pose a lesser threat to Venice than the very thing they come to see — water.

Venice could go under

Rising sea levels are drowning Venice.

Local engineer and scientist Giovanni Cecconi says Venice could be gone in 80 years.

In 50 years the entire ground floor of the city will be under water in the worst-case scenario.

And 30 years after that, buildings would be destroyed.

Venice was flooded by a high tide in November 2018. Images of the city inundated could become more common in the decades ahead. ( AP: Luca Bruno )

"We'll be a ghost town, in the worst-case scenario," he says.

Venice is pinning its hopes on an engineering solution that Mr Cecconi helped design — a $9.6 billion storm surge barrier called MOSE (Modulo Sperimentale Elettromeccanico).

The project is years overdue and was at the heart of one of Italy's biggest corruption scandals.

Venice's former mayor and 34 other officials were arrested for embezzling $30 million of public funds. City authorities hope MOSE will finally be operating later this year.

According to Mr Cecconi, MOSE will be a good stopgap, but not the complete solution.

"Not forever, but it can save Venice for the next 50 years in case of the worst scenario, or for 100," he says.

Engineer and scientist Giovanni Cecconi says rising sea levels could destroy the city. ( Foreign Correspondent: Tim Stevens )

Venice's problems have UNESCO on high alert.

Later this year it will decide whether to put the city on the World Heritage endangered list.

This would allow it to offer immediate financial assistance and advice to the city's caretakers, a move that could force local and national authorities in Italy to make some hard choices about how to save Venice.

"The problem is that we are destroying the city," Mr Cacciari says.

"We are choosing to kill it for money, a little bit of money now, without thinking of what will happen tomorrow."

Watch Saving Venice on Foreign Correspondent at 8pm on ABC TV.