A national park is a reserve of natural, semi-natural, or developed land that a sovereign state declares or owns. Although individual nations designate their own national parks differently (for example, see Martin Luther King, Jr. Memorial in the United States), an international organization, the International Union for Conservation of Nature (IUCN), and its World Commission on Protected Areas, has defined National Parks as its category II type of protected areas. While ideas for this type of national park had been suggested previously, the United States established the first such one, Yellowstone National Park, in 1872. The largest national park in the world meeting the IUCN definition is the Northeast Greenland National Park, which was established in 1974. According to the IUCN, there were 6,555 national parks worldwide in 2006 that meet its criteria.

In 1969 the IUCN (International Union for the Conservation of Nature and Natural Resources) declared a national park to be a relatively large area with particular defining characteristics. A national park was deemed to be a place

A crater lake is a lake that forms in a volcanic crater or caldera, such as a maar; less commonly and with lower association to the term a lake may form in an impact crater caused by a meteorite, or in an artificial explosion caused by humans. Sometimes lakes which form inside calderas are called caldera lakes, but often this distinction is not made. Crater lakes covering active (fumarolic) volcanic vents are sometimes known as volcanic lakes, and the water within them is often acidic, saturated with volcanic gases, and cloudy with a strong greenish color. Lakes located in dormant or extinct volcanoes tend to have fresh water, and the water clarity in such lakes can be exceptional due to the lack of inflowing streams and sediment.

Lava lakes, a rare phenomenon where a volcanic crater can sustainably contain a significant volume of molten lava, are discussed in a separate article.

Crater lakes form as precipitation within the rim fills the created depression. The water level rises until an equilibrium is reached between the rate of incoming and outgoing water. Sources of water loss singly or together, may include evaporation, subsurface seepage, and in places, surface leakage or overflow when the lake level reaches the lowest point on its rim. At such a saddle location, the upper portion of the lake is contained only by its adjacent natural volcanic dam; continued leakage through or surface outflow across the dam can erode its included material, thus lowering lake level until a new equilibrium of water flow, erosion and rock resistance is established. If the volcanic dam portion erodes rapidly or fails catastrophically, the occurrence produces a breakout or outburst flood. With changes in environmental conditions over time, the occurrence of such floods is common to all natural dam types.