Zac Efron's swimming pool incident started when he dove into the water ... the dive was so deep, he hit the bottom of the pool and ingested water into his lungs ... TMZ has learned.
Sources close to the situation tell us Zac went for a swim with friends very early Saturday morning in Ibiza when he decided to take a dive.
We're told Zac ingested a large amount of water into his lungs when his chest hit the bottom of the pool on the dive ... and he showed signs of distress and onsite security helped him out of the water.
Our sources say Zac was momentarily dazed and he was taken to a hospital as a precautionary measure.
During the hospital visit, we're told Zac took a chest x-ray to confirm the water was gone from his lungs. He was cleared for release within hours but remained at the hospital because there were a ton of photogs outside who had gotten wind he was there.
Zac's since posted a photo of himself working out the next day ... and our sources say he went right back to his normal schedule after getting back to his villa from the hospital.
In fact, our sources say Zac went out to dinner Sunday night in Ibiza and returned to the United States today ... and he's back home and healthy.
Sounds like he dodged a bullet!!!