Plain old helium can take balloons on some pretty incredible journeys.
"I eventually got up to altitude, opened the vents up, and was enjoying the nice cold blast of fresh air," wrote a pilot on Reddit. "After about 10-15 minutes, I witnessed a large, swollen, disfigured clown's head fly past the right side of the airplane.
"After determining I was in fact not suffering from hypoxia, I turned around to see what the hell had flown past my airplane.
"My eyes had not deceived me. It was indeed a large, swollen, disfigured clown's head.
"Apparently, someone had let go of their helium-filled party balloon. The pressure differential between the inside of the balloon and the outside atmosphere at 10,000 feet caused it to stretch and distort into an unholy clown mutant from hell."