20/04/2016 11:00 PM AEST | Updated 15/07/2016 12:52 PM AEST

'Google Maps For Underwater' XL Catlin Seaview Survey Captures Coral Bleaching Before And After

When the world's worst coral bleaching event hit the Pacific Ocean this year, a long running underwater visual survey group was ready to record it.

Described as 'Google Maps for underwater', the XL Catlin Seaview Survey had a baseline of healthy reefs around the world, which they could then compare after the 2015-16 bleaching event.

The images and videos are devastating.

Photo gallery XL Catlin Seaview Survey Coral Bleaching See Gallery

Here in Australia, an aerial survey of the Great Barrier Reef found 93 per cent of the reefs were now bleached.

National Coral Bleaching Taskforce convener Terry Hughes said that of 900 reefs surveyed down the 2300km length of the Reef, seven percent was unbleached.

What is coral bleaching?

Coral hosts tiny algae in its tissues called zooxanthellae that produce its food as well as its colour.

When waters become too hot or cold, the coral becomes stressed and its metabolism and reproductive system break down.

At this point, it cannot process the oxygen created by the zooxanthellae, so it expels them.

If the temperature returns to normal, these tiny creatures can re-enter the tissues, or if most but not all algae have been expelled, the remaining few can keep the coral alive.

If the temperatures remain extreme and the sun continues shining, the coral dies.

There is no specific temperature threshold that tips coral into bleaching territory, rather, it's a cumulative stress.

“We’ve never seen anything like this scale of bleaching before. In the northern Great Barrier Reef, it’s like 10 cyclones have come ashore all at once,” Hughes said.

“Towards the southern end, most of the reefs have minor to moderate bleaching and should soon recover.”