The Lyell Glacier, once a mile wide and Yosemite’s largest glacier when measured by John Muir in 1872, could melt off and disappear in as soon as five years, according to park geologist Greg Stock, if warm temperatures at high elevations continue. Chronicle outdoors writer Tom Stienstra visited the park to report on the glacier’s vanishing. This is the trek itinerary.

Day 1: Launch day: In Yosemite National Park, drive to Wilderness Center at Tuolumne Meadows, get Wilderness Permit, continue to trailhead at Dog Lake parking lot near Tuolumne Meadows Lodge, elevation 8,700 feet. Hike 8.5 miles on John Muir Trail, 300-foot elevation gain, up Lyell Fork Tuolumne River; set up camp near where Kuna Creek descends into Lyell Fork, elevation 8,980 feet, acclimate to high altitude. Synopsis: 8.5 miles, 300-foot gain, easy.

Day 2: Trek into cirque: From head of Lyell Canyon, hike John Muir Trail toward Donohue Pass, break off from trail, following a small watershed, and scramble and climb over mostly bedrock slabs into Lyell cirque for base camp at 10,850 feet, above tree line near small unnamed lake, Mount Lyell towering above. Synopsis: 3.6 miles, 1,970-foot climb, mostly off trail, challenging.

Day 3: Maclure Ice Field: From base camp (10,850 feet), climb challenging boulder field to sub ridge, then trek across steep glacier-polished bedrock and talus to Mount Maclure cirque and lake. With ice-climbing equipment, climb up Maclure Glacier and Ice Field into glacier tunnel, 11,554 feet. Synopsis: 2.4 miles, no trails, 700-foot elevation gain, ice climb and return, hazardous.

Day 4: Lyell Glacier: From base camp (10,850 feet), climb boulder field to sub ridge, then trek up ridge across bedrock to small lake and across extremely hazardous loose boulder field and talus across moraine to Lyell Glacier (12,050 feet). With crampons and ice axe, climb up Lyell Glacier to shoulder of Lyell Peak (13,120 feet), open exposure. Synopsis: 3 miles, no trails, 2,000-foot climb, extremely hazardous.

Day 5: Snowbound exit: Blizzard blankets Mount Lyell and adjoining high country, with 6-inch snowfall in three hours. From base camp, canyoneer off-trail descent down drainage, over boulders and talus in snow, low visibility, ford stream, 2.5 miles to John Muir Trail. Descend to Lyell Fork/Tuolumne Meadows, hike out to trailhead in rain/snow. Drive to restaurant, The Mobil, better known as the Whoa Nellie Deli in Lee Vining, for feast. Synopsis: 12 miles, 2,300-foot descent. Extremely hazardous.

Trek totals: 30 miles, 4,970 feet total climb, 10.4 miles off trail across boulder fields, talus, scree and bedrock, six hours ice travel.

Difficulty: The upper sections of the hike are off trail and involve steep, slippery rock scrambling, exposure above cliff bands, very unstable loose rock through the moraine, and slippery ice/glacier travel with crevasse and glacier tunnel navigation.

Disclaimer: “Everyone on the trip needs to assume personal responsibility of their own health and safety.” — Mountaineer Braden Mayfield.

If you want to go: A Wilderness Permit and park-approved bear-resistant food canister are required for overnight travel in the Yosemite Wilderness. Yosemite Conservancy leads Lyell Glacier treks for very fit backpackers each August. Info: www.YosemiteConservancy.org.

Tom Stienstra is The Chronicle’s outdoor writer. E-mail: tstienstra@sfchronicle.com Twitter: @StienstraTom