One of the most prominent features of the interior highlands is a depression that runs from the Caribbean Sea to the Gulf of Fonseca. This depression splits the country's cordilleras into eastern and western parts and provides a relatively easy transportation route across the isthmus. Widest at its northern end near San Pedro Sula, the depression narrows as it follows the upper course of the Río Humuya. Passing first through Comayagua and then through narrow passes south of the city, the depression widens again as it runs along the border of El Salvador into the Gulf of Fonseca.