After exploration by millions of visitors over the past 100 years, it's hard to imagine anything left to discover amid the majesty of Yosemite's glacier-cut granite cliffs and giant Sequoia groves. But, by thinking small, scientists have discovered new species of lichen clinging like microscopic starfish to Yosemite icons such as El Capitan, Half Dome and Vernal Falls, and countless slabs of less famous rock. And they are trying to determine whether the species exist anywhere else.