20,000 items of trash removed from Yellowstone’s geysers and hot springs – including 280 hats and a slew of Pokemon cards

Most thoughtful hikers and campers operate under the "leave no trace" principle — the wilderness should bear no mark of your visit.

Unfortunately, many people aren't thoughtful hikers and campers, so America's wild places — like Yellowstone National Park — end up filled with the junk left behind by tourists.