Invisible Mountain is sort of an odd name because this peak sits right at 12 o'clock in your windshield (windscreen, for Brits) as you are driving toward Mackay from Arco. But it makes more sense when you get to the "trailhead", because you won't see the actual summit until you are about 10 minutes away. You'll think you see it, but that's not it, trust me. The true summit is invisible until you get very, very close.
As stated, you can see it easily from the highway on approach. But presented with the awesome view on the drive, we are always heavy-footed in anticipation, so we keep forgetting to stop to take pictures. There are some really good pictures in the Message Board on IdahoSummits.com, but I feel I should have permission to use those. Perhaps some kind soul will contribute?
One way distance: 3.5 miles
Elevation gain: 4900'
Class: mostly 2, with a 15' section of Class 3
Expected time round trip: 7-10 hours
For most people, all things Lost River start in Arco, Idaho (aka "Atomic City", the first city in the world to be lit by nuclear-powered electricity- Wikipedia) on Highway 93, heading north.
From the intersection in Arco (there is really only one), go 19 miles north to the Pass Creek turnoff, which is well signed. Turn right and head due north for about a mile. The road will angle slightly northeast. In about another 1/4 mile, cross the canal and take the left turn. At the next intersection, turn left (west) on the Bench Road. Go about 2 miles and turn right (north, uphill) just short of the gravel pit. Follow this rougher dirt road (still passable to most vehicles) as you head toward the mountain. Take a steep left turn uphill. This short uphill requires high clearance and perhaps also 4WD low. If you don't have that, it's only about 1/2 mile farther to the end of the road. The short uphill takes you onto the obvious bench. From here, you can see the trailhead, so turn right on a good dirt road. Drive to the end of the flats (the road continues, sort of, but why bother) and park wherever.
Alternately, you can take the highway another 2.25 miles past the Pass Creek turnoff and go due north. This road is not well marked, but takes you directly to the gravel pit mentions previously.
From the "trailhead", proceed up the ridge to the end of the road, or close to it. You can simply follow the crown of the ridge, or you can try angling off to climber's left slightly and try to stay in the grassy areas (as opposed to the brush on the ridge crest). As you move higher, you need to be on the west side of the ridge; the east side involves lots of terraced limestone cliffs.
At about 9000', you will climb above timberline. Follow the ridge as it twists back and forth along the huge cirque just to your right (east). It's usually easier to stay below the crest where it is less rocky, but follow your nose.
At about 10,800', the ridge narrows into a crest, forcing you to down-climb about 15' of Class 3. The goats do this on climber's right. If that looks bad, move climber's left and work your way down several easy but exposed ledges.
From there, continue following the arcing ridge as it continues to curl northeast and eventually due east.
No registration. No gates on the road. But this is really wild, lonesome country, so if you screw up and no one knows where you are, you might be there forever.
The nearest sanctioned camping nearby would be at Mackay Reservoir, north of the turnoff from the highway.
If you camp at the "trailhead", you won't find any amenities. You will find lots of cowpies.
External LinksMackay info
Splattski trip report
Lost River Ranger District, Salmon-Challis National Forest
Gib's Borah cam, looking at the Lost River range about 20 miles north
For additional information on this climb and other peaks in the area, please see Tom Lopez's excellent book, Idaho: A Climbing Guide.