| In this Sunday May 21, 2006 file photo, a tour bus leaves the Wilderness Access Center inside the Alaska's Denali National Park. For years, visitors who wished to see Denali National Park's grizzly bears, moose, sheep and caribou have had to ride diesel buses that spew carbon dioxide, nitrogen oxide and particulate matter into the air. But park officials are testing a new hybrid bus that promises to run cleaner and cheaper. (AP Photo/ Al Grillo) |