The Þjórsá is Iceland's longest and mightiest river, and its valley -- Þjórsárdalur -- holds what is believed to be the world's most voluminous lava flow since the end of the last Ice Age. The flow is about 8,000 years old, and covers around 805 sq. km (499 sq. miles). In the river's lower reaches, the lava has yielded to fertile grazing land. The upper reaches near Hekla remain bleak, thanks to ash fallout from Hekla's periodic eruptions.