What are these balloons doing in Yosemite? These pictures were all taken at off-trail locations in the park wilderness, miles from any developed area. When helium filled balloons escape from parties or graduations, they rise quickly into the atmosphere, sometime reaching heights of five miles. At this point they either burst and fall to earth or get carried by the wind until they slowly deflate and sink. Either way, they can travel large distances, including into the backcountry of Yosemite. Latex balloons will ultimately biodegrade, but Mylar - or more correctly, metalized nylon - balloons, ribbons, and other plastic items will take hundreds of years to break down (and even then they won't decompose, only becoming microscopic pieces of plastic). The next time you are at a party, remember Yosemite and hang on tight to that balloon!