The one that pops into my mind is the Aqua Fria cache. My first try was by four wheel drive from the north. At the time, the lake water was up so high that there was no way to get a land vehicle anywhere near the cache. My second try was from the south by hiking in on wild burro trails. Got to within 1.25 miles of it and came to a small version of the Grand Canyon. By that time I had less than half my water supply left and had to turn back. I actually did run out of drinking water, but had a fresh gallon in the truck when I got back to it. In the mean time, the clue I wanted from that cache to solve the Chain Cache, had been changed to a more accessible cache. I still haven't gone back to get the Aqua Fria cache, but would like to when it is accessible by Jeep.