During the Pleistocene ice ages, Mauna Kea's summit was capped by glaciers that extended down its flanks, carving moraines and polishing rock surfaces that remain visible today. The reserve, established in 1981 and administered by the Hawaii Department of Land and Natural Resources, spans elevations from roughly 10,000 to 13,000 feet on the mountain's southern slope, encompassing the zone where glacial evidence is best preserved. Above the reserve sits the summit complex of Mauna Kea Observatory; in 1998, the observatory's lease area was specifically redrawn to exclude the Ice Age reserve, acknowledging the geological significance of the terrain. The reserve lies above the Onizuka Center for International Astronomy, named for Ellison Onizuka, the Hawaii-born astronaut who died in the Challenger disaster, and it is accessed from the Saddle Road, about 24 miles northwest of Hilo.

Ancient Hawaiians climbed to over 12,000 feet on Mauna Kea to quarry a dense, fine-grained basalt that could be shaped into adze blades, the primary cutting tool of Polynesian civilization. The Mauna Kea Adz Quarry is the largest primitive quarry in the world, a sprawling site of workshop floors, flaking debris, and partially finished tools scattered across the volcanic landscape near the summit. The quarry required extraordinary commitment from its workers: at this altitude, temperatures drop below freezing, the air holds roughly 40 percent less oxygen than at sea level, and the terrain offers no shelter. That the quarry was used extensively over centuries speaks to the exceptional quality of the stone, which was worth the physical cost of extracting it. On December 29, 1962, the quarry was designated a National Historic Landmark.

The reserve harbors one of the most improbable creatures in the Hawaiian Islands. The wekiu bug, Nysius wekiuicola, is a seed bug that has adapted to life above the tree line on Mauna Kea's summit, feeding on insects blown up the mountain by wind currents and killed by the cold. A 2004 survey confirmed the wekiu bug's presence within the reserve, where it inhabits the rocky crevices of cinder cones at elevations where few other arthropods survive. The bug's name comes from the Hawaiian word for "summit," and its existence illustrates evolution's capacity to colonize even the most hostile environments. Lake Waiau, one of the highest lakes in the United States at approximately 13,020 feet, also sits within the reserve's boundaries, a small, shallow body of water sustained by permafrost and precipitation in a landscape where standing water seems almost impossible.

The reserve occupies a peculiar position in the cultural and scientific geography of Mauna Kea. Below it, the Onizuka Center serves as the staging area for astronomers ascending to the world-class observatories above. Above it, the summit hosts some of the most powerful telescopes on Earth, instruments that have expanded humanity's understanding of the cosmos. Between these two zones of modern science, the Ice Age reserve preserves evidence of far older forms of knowledge: the glacial geology that reveals Hawaii's Pleistocene climate, and the quarry that records how Native Hawaiians understood their own landscape well enough to travel to extreme altitudes for the best stone. The reserve was carved out precisely to prevent either the observatory complex or other development from erasing this record, a 3,894-acre buffer between the deep past and the technological present.