From Wikipedia Picture Of The Day; August 19, 2013:
A lava lake at Mount Nyiragongo, a volcano found in Virunga National Park in the Democratic Republic of the Congo. Lava lakes, which can form in three different ways, are large volumes of molten lava, usually basaltic, contained in a volcanic vent, crater, or broad depression. Persistent lava lakes such as the one at Nyiragongo, which is the largest to appear in recent times, are rare.
Photograph: Cai Tjeenk Willink