As the World Heritage-listed natural wonder recovers from life-threatening coral bleaching outbreaks, the Great Barrier Reef spawns in an explosion of color.

On Tuesday evening, scientists observed the corals produce billions of children as they discharged sperm and eggs into the Pacific Ocean off the coast of Cairns, Queensland.

The spawning event lasts two or three days each year.

In 2016, 2017, and last year, the 2,500 reef system, which covers 348,000 square kilometers (134,000 square miles), was severely affected by coral bleaching caused by extremely warm ocean temperatures. Two-thirds of the coral died as a result of the bleaching.

As part of a reef health monitoring initiative, Gareth Phillips, a marine biologist with Reef Teach, a tourist and education firm, is investigating spawning.

"Seeing the reef give birth is rewarding," Phillips said in a statement on Wednesday. "This is a solid proof that its ecological functions are intact and functioning after being in a container for a long time."