The 'Gap'

 About 200 miles south of the Panamanian border....on the Pacific side...you come to the last bit of civilization in Columbia.  Anything west of Medellin (figure about 25 miles) is a territory that you'd be advised to just skip or avoid.  

This jungle heads northward (for about 200 miles) then reaches the Panama border, and there you find the Darien Gap....which is another entire jungle.  In this case (on the Panama side) you can figure another 30-odd miles of absolute pure jungle exists.

The Trans-American Highway?  It ends at Yaviza....a small town about forty miles from the Columbian border.  The idea of building the 'link' of the highway through the jungle.  It was discussed years ago, and eventually dropped.  

On the Atlantic side....there's a civilized area near the coast, and if you had a boat....you could probably make it across the 'bay' to civilized Panama (maybe a 80-mile sailing adventure). 

What keeps a vast number of migrants in South America from venturing the US border?  Well....basically the Darien Gap.  Some people say they venture through trails in the jungle.  The story goes (depending on who tells it), that maybe a couple thousand attempt this year, and maybe 80-percent make it....with the rest dying or disappearing somewhere in the jungle.  

On the top ten most dangerous travels you could make in the world....I'd easily rate this in the top ten.  Beyond the snakes and critters.....you have to worry about various fevers which are existent there.  On top of that, you have to cross swamps, creeks, streams, and deal with waterfalls.