now to california's unprecedented drought. and some of its silent victims. giant sequoia trees. they can live to be 3,000 years old. but new evidence shows they are struggling with the drought more than ever. now a team of scientists is taking the trek up to monitor them. nbc's gaddy schwartz went along for the climb. >> reporter: towering above the sequoia national forest, living icons of resilience that have survived fire, storms, and insect infestations. attracting thousands to go stand next to their massive trunks. but in this historic drought, something is happening to the mighty sequoias. >> this area was identified as having the highest number of trees that are showing symptoms of what we think is drought stress. >> reporter: some of these trees have lost 75% of their needles. that's what you're seeing down here on the forest floor. but to see the true stress of