The vast crater of Haleakala is 900 metres deep, 43 km in circumference, and is not actually a true crater, in the volcanic sense. It is a high-altitude, water-eroded depression carved out by two major stream systems, then subsequently partially filled by cinder cones and lava flows to create an other-worldly landscape of basaltic dikes, lava tubes, vents, and rock and cinder deserts.

Though at first glance the terrain may appear barren and forbidding, the summit and crater of Haleakala are home to a diversity of rare plant and animal communities, including high-altitude tussock grasslands, alpine shrublands, and the alpine desert habitat of the renowned (and threatened) Haleakala silversword, Argroxiphium sandwicense.

Threatened birds found within Haleakala National Park include the Hawaiian goose, or nene; the Maui akepa; the Maui parrotbill; the Maui nukupuu; and the crested honeycreeper, among others.

The rainforests of the Kipahulu Valley are among the most pristine in the state of Hawaii, as they are extremely dense and occur in a treacherous environment of abrupt gulches and very high rainfall. Many rare plants occur here.

Haleakala, originally part of Hawaii Volcanoes National Park, was re-designated as a separate entity in July 1961. Haleakala National Park was designated an International Biosphere Reserve in 1980.