While the U.S. government focuses on protecting the Mexican border with extra federal agents, fences and sophisticated technology, the much larger northern border remains porous and highly vulnerable to crossings by terrorists and other extremist groups.

That’s because the nation’s vast border with Canada is the world’s longest (nearly 4,000 miles of land and water) open border with dozens of unmanned crossings and nearly 60 wide open roads and trails leading in and out of the U.S. This offers a multitude of opportunities for clandestine and undocumented crossings, according to a Department of Homeland Security report presented to Congress this week.

The 20-page report points out that, despite the terrain’s vastness, law enforcement on the immediate border is limited even though more than 70 million international travelers and 35 million vehicles cross it annually. This allows extremists and their conveyances to enter the U.S. undetected because there is an undisputed presence in Canada of known terrorist affiliate and extremist groups, including Hezbollah, Hamas and the Armed Islamic Group of Algeria.

Despite the documented northern threats, the area has less than half the Border Patrol sectors protecting it than the Mexican border, which spans about 2,000 miles. Officials say the discrepancy is due to the major differences in the terrain, with the north ranging from densely forested lands to open plains, including the Great Lakes Region, and spanning from Washington through Maine.

The alarming Homeland Security report was required by Congress under the 9/11 Commission Act of 2007, which demands that the agency submit detailed information regarding initiatives to improve security along the northern border. Nearly seven years after the worst terrorist attacks in the nation’s history, little has changed to protect the increasingly vulnerable U.S. crossing.