So why has Canada managed to sustain popular acceptance and cross-party support for so much legal immigration? For one thing, the movement of people into the country has generally been so law-abiding and orderly as to be uncontroversial and barely newsworthy. Canada, unlike the U.S., is a country where nearly all arrivals come in through the front door, in the open, during daylight hours.

Almost everyone who immigrates to Canada has to first apply from overseas, and before they’re granted entry they’re subjected to extensive vetting by Canadian authorities. Those who make the cut have to wait months or years for their turn in line before being let in. Over the past 20 years, about 5 million immigrants chose Canada. But the vast majority only entered the country after Canada also chose them.

As for illegal and irregular immigration, Canadian governments from both ends of the political spectrum have worked—quietly—to ensure there is as little of it as possible. The unspoken underpinning of Canada’s otherwise welcoming immigration policy is a giant and assiduously maintained border wall.

Wait, what? Yes, Canada has a border wall—in a sense. In fact, it has five of them. Four are geographic, the fifth is bureaucratic. All have been extremely effective in sustaining the legitimacy and popularity of Canada’s immigration policy.

Three of the walls are the dumb luck of geography: the Atlantic, Pacific, and Arctic oceans. You can cross the Aegean from Asia to Europe in a dinghy, but unless you can get your hands on a ship and a crew trained in navigating thousands of miles of difficult water, you aren’t sailing to Canada. So far in 2018, Canada has received exactly 10 asylum applications at sea ports.

The fourth wall is Canada’s southern border with the U.S. The world’s leading economy has historically been a magnet for people, not the reverse. In the late 19th and early 20th century, the volume of emigrants from Canada to the U.S. was at times so high that Canadians actually feared for the future of their country. The strength of the American economy long meant that few immigrants would think to use the U.S. as a back door into Canada.

The fifth wall is the bureaucratic barrier that Canadian governments, both Conservative and Liberal, have meticulously maintained to cover any gaps in the other defenses.

Toronto’s Pearson International Airport is one of North America’s busiest hubs, with direct flights from nearly every continent. But if you want to come to Canada, it’s obviously not as simple as buying a plane ticket. Visitors from a long list of countries need to apply in advance for a visa, and if they’re coming from a place that has recently sent Canada more than a certain number of asylum applicants or vacation over-stayers, that visa may suddenly become difficult to get. And without a visa, the airline won’t let you on the plane.