The Río Hondo in northern Belize is the border with our neighbor Mexico. The river has its headwaters in Guatemala, where it is known as the Río Azul, or Blue Creek in Belize. The river runs 230 kilometres from its headwaters into the Bay of Chetumal, known in Belize as the Corozal Bay. Interestingly, Río Azul runs in a southerly direction from Guatemala into Belize, but then takes a determinedly sharp turn to the north at La Unión where it becomes the Río Hondo, making for Belize's characteristic natural border with Mexico. This video from Mileneo is in Spanish and gives an excellent tour of the river.