Hiking on Mauna Kea means high altitude hiking. Although the height of the mountain (13,796 feet) is not necessarily a problem, the elevation gain in a short hour or two of getting to the top is. It takes time for the body to acclimatize, and when you drive up from the ocean you rob yourself of the chance to acclimatize easily.
What you may expect to experience normally are slight dizziness, a shortness of breath due to reduced oxygen levels, and reduced ability to think clearly and react quickly. Some people are more prone to elevation problems, so if you experience more severe symptoms, get to a lower elevation immediately! These symptoms include prolonged or severe headache, loss of appetite, cramped muscles, prolonged malaise or weakness, dizziness, reduced muscle control and balance, and heart palpitations. Use your head, know your limits, and don’t push yourself. Carry plenty of water (more than you would at a lower elevation) and food. Wear a brimmed hat, sunglasses, sunscreen, and lip balm, a long-sleeved shirt and long pants, and sturdy hiking boots or shoes. Carry a jacket, sweater, and gloves, as it can be cold and windy at and near the top. Don’t alter the natural environment and stay on established trails.
There are a few good trails on the mountain. About six miles above the Onizuka Center for International Astronomy Visitors Information Center, a dirt track heads off the access road to the west and downhill to a parking lot. From the parking area, it’s about one mile farther west, over the saddle between two small cones, to Lake Waiau and its placid waters. This should take less than 30 minutes.
On the way, you cross the Mauna Kea Humu‘ula Trail, which starts at the third parking lot near the T intersection above and heads down the mountain to the visitors center. Taking the Humu‘ula Trail to Lake Waiau should also take about 30 minutes. Continuing on down the Humu‘ula Trail a couple of miles brings you past an ancient adze quarry site. Perhaps the most convenient hike is that to the true summit of the mountain. Start from the roadway across from the University of Hawai‘i’s 2.2-meter telescope, cross over the guardrail, and follow the rough path down into the saddle and steeply up the hill, a distance of less than half a mile.