From the summit one looks down into a crater some 11.25 km (7 mi) across, 3.2 km (2 mi) wide, and nearly 0.8 km (0.5 mi) deep. The surrounding walls are steep and the interior mostly barren-looking with a scattering of volcanic cones. It has been called the largest extinct volcanic crater in the world, but this is not at all accurate. Macdonald and Abbott (1970, p. 329-330) state it this way:
Surrounding and including the Haleakala crater is Haleakala National Park, a 28,655 acre park of which 19,270 acres are wilderness. The temperature near the top of Haleakala tends to vary between about 40 degrees fahrenheit and 60 degrees fahrenheit and, especially given the thin air and the possibility of dehydration at that elevation, the walking trails can be more challenging than one might expect. There are two main trails leading in from the summit: Sliding Sands Trail and Halemau'u Trail.
Because of the remarkable clarity, dryness, and stillness of the air, and being above one-third of the atmosphere, the summit of Haleakala on Maui (like Mauna Kea) is one of the most sought-after locations in the world for ground-based telescopes and is an important obervational centre with the associated computing facilities and expertise that this requires. Some of the telescopes are operated by the US Department of Defense and involved in researching man-made (e.g. space craft, satellites, rockets) rather than celestial objects. The astronomers on Haleakala are concerned about increasing light pollution as Maui's population grows.