In this amazing photo we see the lava lake at Mount Nyiragongo, an active stratovolcano inside Virunga National Park, in the Democratic Republic of the Congo. The volcano has erupted at least 34 times since 1882, creating various-sized lava lakes that have formed, cratered, drained and reformed. The image above (taken in 2011) shows the most recent iteration of the mountain’s lava lake. It was formed from a 2002 eruption. [Source]

Lava lakes are large volumes of molten lava, usually basaltic, contained in a volcanic vent, crater, or broad depression. The term is used to describe both lava lakes that are wholly or partly molten and those that are solidified (sometimes referred to as frozen lava lakes). [Source]

Lava lakes can form in three ways:

1. From one or more vents in a crater that erupts enough lava to partially fill the crater
2. When lava pours into a crater or broad depression and partially fills the crater
3. Atop a new vent that erupts lava continuously for a period of several weeks or more and slowly builds a crater progressively higher than the surrounding ground

The lava lake at Mount Nyiragongo is one of only four persistent/near-persistent lava lakes in recent decades. [Source]