Probably one of the coolest things I've ever seen was at the Delaware Water Gap,NJ. I was hiking with some fellas I had met there the day before, we had a little party, and were all a little fuzzy. Were hiking along and all of a sudden there was this whistle that filled the Woods, it got louder and louder, then someone asked "What the heck is that" I had heard this sound before, but was drawing a momentary auditory blank, then it hit me, quick, and I practically pulled a muscle in my neck as all in one motion I looked up, pointed, and said "there, right there" it was a sail plane (glider) about 50 feet above the trees, it was early spring, and the leaves weren't out yet, the plane was so close I could see the pilots sun glassed face as he looked right at us, again, one of the coolest things I've ever seen.
When the westerlies blow just right, Sail planes fly the ridge of the Kittatinnies in NJ and can cross into PA and follow the Blue mountains all the way down to about MD turn around and come back, this is called or classified as a cross country flight because of the distance involved, there is an airport near the AT in Blairstown NJ, thats where I took my first sail plane ride, and later logged about 6 hrs in a Schweizer 233 trainer.
Next time you here this whistle while hiking, don't forget to keep looking up, very cool indeed.