The Darién gap is a 99 mile long by 31 mile wide area completely covered in swampland and forest that separates Panama's Darién province with Colombia. Constructing in the Darién gap would have extremely high costs and immeasurable ecologic impact, which is why it remains undeveloped.
However, this small gap is the only thing that prevents the Pan-American Highway from connecting all three continents. If you measure a straight line between where the highway stops in Panama and starts in Colombia, there's a 62 mile gap between the roads.
There have been plans to remedy this missing link in the Pan-American highway since 1971, but they have been halted because of the extensive environmental damage this would cause. There is evidence that this gap has prevented the spread of foot-and-mouth disease to Central and North America, among other factors.