The U.S. Navy says the planes had to abandon the bombs because they were running out of fuel and feared wouldn't be able to land with them on board. Two of the bombs were non-explosive, the Navy says. Two others were disarmed before they were dropped. They did not explode, according to the Navy.
Officials say the pilots dropped the bombs in an area away from the reefs and deep enough where ships won't run into them.
The great barrier reef is home to thousands of animal species.
It's a habitat for animals threatened by extinction.