Image copyright Getty Images Image caption Demonstrators protest against Chicago's bid to host the 2016 Olympics

It has long been billed as the greatest show on Earth.

Both the summer and winter versions of the Olympic Games are multi-billion dollar spectacles that bring together the world's top athletes and draw masses of media coverage.

So when residents in the Canadian city of Calgary voted on whether to bid to host the 2026 Winter Olympics last week, you might have expected an enthusiastic thumbs-up.

But instead their verdict was clear: thanks, but no thanks.

Their rejection, motivated by fears about high costs and questions about the economic benefits, came after three other cities all withdrew from bidding earlier this year.

This is part of a bigger problem that experts say could threaten the very future of the Olympic Games: fewer and fewer cities around the world want to play host.

Just consider the numbers: the 2004 Summer Games, which were ultimately held in Athens, attracted 11 bids but the 2024 event garnered just two.

So what's behind this trend? Chris Dempsey knows more than most about why there is increasing scepticism.

He spearheaded the opposition to Boston's bid for the 2024 Olympics, a movement that started in his living room in early 2013 after a conversation with some friends.

"We saw that a group of very wealthy and powerful [people] had gotten together and decided that a bid would be a good thing for Boston," he says. "But we were concerned about the cost."

"At first, the polling showed that Bostonians were very open to hosting the Olympics," he explains. "But when they started reading the fine print - that's when they decided that the bid wasn't in their best interests."

Image copyright Getty Images Image caption Chris Dempsey led a grassroots campaign to oppose the Boston bid for the 2024 Olympics

He says the financial burden of hosting the Games was the primary concern, along with a resentment of outsiders telling the city what would be best for it.

"[We] wanted to be heard and to chart the future of our city. I think in Calgary they felt the same way," he says.

And he appears to have a point.

After Calgary residents voted overwhelmingly against running as hosts last week, one Calgary councillor, Sean Chu, said: "I think that people had enough of the establishment, telling us what to do, what to think. They tell you to spend millions, billions, it's good for you."

After polls began to show declining support for Boston's bid, it was withdrawn in the summer of 2015 in what was seen as an unlikely victory for the 'No' camp.

"The 'Yes' side spent about $15 million (£11.7m) and we spent less than $10,000 on our entire campaign," Mr Dempsey says.

He believes that residents should assess the facts of an Olympic bid rather than just buying into the pride and honour of being a host.

"When democracies look at this as a public policy question - and they soberly assess the pros and cons - their voters are realising that this is just not in their best interests," he says.

"We were always confident that we had the facts on our side."

Growing concerns over the economic benefits of hosting an Olympics are not unreasonable, experts say.

"There is good reason for cities to be concerned," says Professor Bent Flyvbjerg, who has studied decades of Olympic budgets. "The general trend is that costs have been going up and governments are anxious about spending too much money.

"It's a multi-billion dollar budget that you will need to host the Games and that's only covering the direct sports-related costs," he says. "There are also the indirect infrastructure costs such as improving transport systems."

The cost of the 2014 Winter Games in Russia's Sochi was put at $51bn - the most expensive in history. The Beijing Summer Olympics in 2008 cost $40bn.

Prof Flyvbjerg says that every Olympic Games since 1960 has run over its budget - some by startling amounts.

Image copyright BBC Sport

So why is spending increasing? "People like to put on bigger parties," he says. "There's a sort of competition that everyone wants to do the best Olympics ever."

The other reason for this is inexperience, Prof Flyvbjerg suggests.

"Any city that puts on the Olympics has never done it before or did it so long ago they cannot use that experience," he says. "So you have inexperienced people in charge of putting on a multi-billion dollar event which is bound to lead to cost overrun.

"It's not sustainable. I think it's come as a bit of a shock for the International Olympic Committee [IOC]."

The IOC, which organises the event, does recognise the worry around costs.

"We know the cost of the Games is a concern," its Olympic Games Executive Director Christophe Dubi tells the BBC. "I feel that we have to make all efforts to contain the costs and complexity."

He says that a number of measures have been introduced in recent years that are aimed at reducing the cost of hosting the Games.

The organisation has started pledging money to offset some of the financial burden. It has promised to contribute $1.8bn to the organising committee of the 2028 Games in Los Angeles, for example.

It also wants to see bids from cities that already have the right infrastructure and venues in place. This is due to growing concerns about so-called "white elephants", expensive facilities that go to waste after the Games have finished.

Take this year's Winter Olympics in Pyeongchang in South Korea. The centrepiece of that event was a brand new $109m stadium that was used a grand total of four times.

In Brazil, many of the expensive arenas that were used in the 2016 Olympics have been left abandoned, creating environmental hazards.

Image copyright Getty Images Image caption Before and after: The Aquatics stadium was left abandoned after the 2016 Games in Rio

"Generally speaking if a city doesn't have a stadium or arena before the Olympics come, then why would they need one afterwards?" asks Professor Andrew Zimbalist, a sports economist who was also part of the No Boston Olympics campaign.

"The reason why the stadium or arena didn't exist before the Olympics happened is because there wasn't an economically viable reason... to make the investment," he says.

"It ends up being financially and environmentally very wasteful."

All of this contributes to a view that the Games are excessive and extravagant. This is something the IOC says it is trying to combat, and Prof Zimbalist suggests that seeking bids from cities with pre-existing arenas is the right approach going forward.

"Some cities like Los Angeles can get away with hosting the Summer Games with almost no investment so then it can make sense," he says.

So why would a city without the right infrastructure bid at all?

Daniel Ritterband, who worked in marketing for the London 2012 Olympics and on the Budapest bid for 2024, argues that the Games can actually have a positive impact on less-developed cities.

Image copyright Getty Images Image caption Fireworks go off over the Pyeongchang Olympic Stadium

"I think the future is for small cities of a million people where the Games can really have a transformative effect," he says. "Some of the things that are disruptive in bigger cities - like specialist traffic lanes - are less disruptive in smaller ones."

He says that having public support is "essential". "If you don't take them with you then you're doomed - that's what happened in Boston," he says.

So what would his advice be to a city considering a bid in future?

"It needs to be part of a 20 year plan of how to move the economy onwards where you build up the infrastructure and support then it's totally worthwhile.

"There has to be a clear benefit to the people in terms of jobs and skills," he adds. "Whether that's hospitality programmes or the increase in short-term contracts.

"It's the greatest moment of national pride a country can have. How people were talking about it in 2012 showed that the nation had their chests puffed out."

Image copyright AFP Image caption A Londoner celebrates as the announcement is made that London will host the 2012 Games

But the reality is that bids from smaller cities, however beneficial an Olympics may be, are not being made. So if the worst-case scenario for the IOC comes and the bids dry up, what can they do?

"The rational end is to have one city that's the permanent host of the summer Games and one that is the permanent host of the winter Games," Prof Zimbalist says.

Mr Dempsey says "this could make a lot of sense", but there would still be questions about who would take up the site indefinitely.

"The problem is that these cities don't really show an interest in taking them now," says Professor Donald Dengel, who has researched the long-term impact of the Olympics on a host city.

"So the IOC would have to contribute... to the upkeep and maintenance of the facilities so the citizens don't have to take up that burden."

"They're going to have to make the financial incentives much greater," he says. "It's a really tough call for them."