The Highlands of Iceland cover most of the interior of Iceland. They are situated above 400–500 metres and are mostly an uninhabitable volcanic desert, because the water precipitating as rain or snow infiltrates so quickly into the ground that it is unavailable for plant growth, which results largely in a surface of grey, black or brown earth, lava and volcanic ashes. A few oasis-like areas, such as Herðubreiðarlindir near Askja, are found only in proximity to rivers.