Two energy companies, Dominion and Duke, want to build a pipeline underneath the famed Appalachian Trail, but environmentalists want them to go take a hike. On this episode of Parts Per Billion, we head out to the trail with Bloomberg Environment's Ellen M. Gilmer and find out what this dispute is about and why it's heading all the way to the Supreme Court. Host: David Schultz. Editors: Marissa Horn and Josh Block.