A 30-foot-by-3-foot solar-powered balloon got away from students conducting an experiment on the Pacifica High School campus shortly after 12 noon Wednesday.
The Federal Aviation Administration reported the balloon was approximately 2,000 feet high and heading toward John Wayne Airport.
Authorities were alerted to the situation, according to an FAA spokesman.
It was part of an advanced-placement environmental sciences class. The balloon was tied down with a string. As the temperature heated up, the balloon broke loose around noon.
"We ran through the halls to go find it, to catch it on the other side," said Pacifica science teacher Steve Doucette. "And by the time we got over there, in just a matter of seconds, the balloon was 500 feet into the air, and we're all just standing there going, 'Oh my gosh.' And this balloon, to give you an idea of how long it is ... it's the entire length of the quad. This thing is huge."
As the temperature decreases as the sun goes down, Doucette expects the balloon will start to descend.
CORRECTION: ABC7.com earlier reported Garden Grove High School launched the balloon. The balloon originated from Pacifica High in Garden Grove.
