Start this route from the McCullough Gulch Trailhead at 11,080 feet. The trailhead is located on the east side of the Tenmile Range. From the new trailhead accessing Quandary's East Ridge, continue on Forest Service 851 until the road splits. Veer to the left here and continue driving until you reach the gate. Park here. You should be able to get to this trailhead with any vehicle. Atlantic Peak will capture your attention once you obtain the split in the road.
The Atlantis Couloir Route is 6.4 miles roundtrip with 2,761 feet of elevation gain. Since this is a non-fourteener route, I consider this a hidden gem in the Tenmile Range.
From the trailhead, the rocky 4wd road will quickly climb past some private property. After a half to three-quarters of a mile, it will turn to a singletrack trail and will meander through groves of pine trees. After a small stream crossing, the trail will turn south toward Quandary and crest a small ridge. Here you have a choice of descending toward a small lake and continuing straight up the valley towards Atlantic or gaining another tiny crest on the valley's lefthand side. Whichever option you choose, head towards the giant snow "V" on Atlantic's east face. I would suggest gaining this crest. This option will allow you to avoid the bushwhack through some willows and will lessen the amount of postholing you may encounter on the valley floor.
When you arrive at the base of Atlantic's east face at 12,800 feet, you will be greeted with your first glimpse of Atlantis. It is the gully on the north(right) side of the "V" couloir. Ascend the couloir to a split at 13,350 feet. The easier and right branch continues straight up to the summit ridge at 13,440 feet while the steeper(more fun), left branch travels to meet the rocky summit block at approximately 13,500 feet. From the top of either couloir, ascend southwest up the easy albeit rocky summit block to Atlantic's summit at 13,841 feet. The summit register is located at the southwest corner of the summit plateau.
I would not suggest this route after the snow has melted because it's an unpleasant, rockfilled, scree gully.
For this climb, I would consider my crampons and ice axe to be essential. You may want snowshoes if your approach will take you straight through the valley. No ropes or pro is necessary here. Climb Atlantis anytime during the spring snow season, May through July. It is a southeast facing couloir which you can preview from Quandary's summit.
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