Several large rivers define Bulgaria's geography and historical landscape. The mighty Danube has constituted the northern border of the Bulgarian territories for centuries. The Iskar bypasses Sofia (locals still jokingly declare it to be the deepest river in the world) and then carves its way into the Stara Planina gorge, oft-described in the late-19th century Bulgarian literature. The Maritsa flows through the Thracian Plain: an ancient route used by generations of invaders, merchants, emissaries and empires. Today, the international route E80 runs along much of its course.