CBS News has learned the number of people caught illegally crossing the United States' northern border is up 142 percent. Border Patrol agents apprehended 445 people entering the U.S. from Canada during the first six months of 2018; that compares with 184 during the same period last year.

The northern border is very easy to cross, which was actually the original intention. But it also makes it very difficult to secure.

An unguarded metal fence is the only thing keeping smugglers from entering a remote corner of New York State – a small part of the physical security infrastructure along America's northern border.

Norm Lague, the Border Patrol agent in charge here, says it's impossible to cover 100 percent of the entire 5,525-mile-long U.S.-Canadian border, the longest and busiest land boundary in the world.

Border Patrol Agent Norm Lague shows Don Dahler part of the border protection between the U.S. and Canada. CBS News

"It's a tough challenge to go ahead and take the limited resources we have and work in such a vast area," Lague told correspondent Don Dahler

About 400,000 people and more than 1.6 billion in goods legally cross the border every day.

Last year, Border Patrol agents along the northern border apprehended 3,027 people who were in the country illegally. By comparison, agents along the southern border captured more than 300,000.

"The northern border is much more vast," said Lague. "The terrain is very difficult to work in, and we do not have the resources at our disposal that the southern border has," said Lague.

Notably, of those apprehended along the northern border, nearly half (1,489) were from Mexico. Mexican citizens don't need a visa to enter Canada, and one-way flights to Toronto and Montreal only cost about $300. Once there, Border Patrol agents say, they can slip into the U.S.

Many buildings and businesses literally straddle the northern border. At Derby Line, Vermont, a row of flower pots marks the northern border, which actually cuts right through the Haskell Free Library and Opera House.

The border between Derby Line, Vt., and Stanstead, Quebec actually cuts right through buildings, and is demarcated by a row of flower pots. CBS News

And illegal traffic goes both ways. Just last January, a Canadian man pleaded guilty to smuggling handguns through the Haskell library into Canada.

Many official ports of entry, like a floating dock near Lake Champlain, are unmanned at night. Border Patrol agents rely on local residents, patrols and sensors to alert them to possible crossings.

Thousands of sensors deployed along the border incorporate motion detectors and cameras. When they sense someone coming across, the tricky part is getting a border patrol agent there in time quickly enough to catch them.

Sensors dot the U.S.-Canada border. CBS News

Sometimes entering the U.S. is as simple as walking across a 20-foot wide clearing in the woods, or paddling across a lake. And you don't have to check in with anyone.

Porter Fox is author of "Northland: A 4,000-Mile Journey Along America's Forgotten Border" (W.W. Norton). "It's generally agreed that the northern border is more vulnerable to a terrorist sneaking into this country," Fox said. "The only known terrorists to be apprehended coming overland into America came from the North.

"This border was created in a different time. It was created as the world's friendliest border between two countries."

He said that seven million American jobs depend on cross-border business. "There's a lot at stake up here in the North," Fox said.