The Isthmus of Panama, also historically known as the Isthmus of Darien, is the narrow strip of land that lies between the Caribbean Sea and the Pacific Ocean, linking North and South America. It contains the country of Panama and the Panama Canal. Like many isthmuses, it is a location of great strategic value. 

The isthmus was formed some 3 million years ago during the Pliocene epoch. This major geological event separated the Atlantic and Pacific Oceans and caused the creation of the Gulf Stream.  http://en.wikipedia.org/wiki/Isthmus_of_Panama