Granite spires and domes rising 60 stories from a high desert basin in southern Idaho created a landmark that California Trail emigrants called the "City of Rocks" in the 1840s and 1850s; at Register Rock, they inscribed their names in axle grease on the stone, and some of those signatures survive today. The reserve is now also a premier technical rock climbing destination, with over 600 established routes on the otherworldly granite formations.