【I Am a Plaything】

Australia's Great Barrier Reef is I Am a Playthingone of the world's great wonders, but scientists have been surprised to find something equally spectacular lurking behind it.

A team from James Cook University, the University of Sydney and the Queensland University of Technology have uncovered an ancient, little known reef off the coast of northern Queensland.

Using LiDAR technology, the researchers were able to map vast fields of the mysterious reef, populated by giant donut-shaped rings. Known as Halimedabioherms, the rings are around 200 to 300 metres (656 to 984 feet) wide and up to 20 metres (66 feet) thick. Because they sit in deeper water, people boating or diving on the reef are unlikely to notice them.

Mardi McNeil, geoscience researcher at the Queensland University of Technology, told Mashablescientists have known about presence of Halimedabioherms on the Great Barrier Reef, but their sheer extent came as a shock.

"We've now mapped the Halimedabioherms to cover just over 6,000 square kilometres (1.5 million acres) on the outer shelf of the Great Barrier Reef," she said. "The previous estimate was about 2,000 square kilometres (494,211 acres). So really, it's three times as large as previously thought."

Stretching up into the Torres Strait, the bioherms sit alongside the outer barrier coral reefs at the edge of the Great Barrier Reef.

The bioherms are built by a calcareous green algae called Halimeda. This algae has a calcium carbonate skeleton that as it grows, breaks off little fragments of skeleton that create the bioherm structure. McNeil compared them to corn flakes.

The team estimate the bioherms have been built over the past 10,000 years, thanks to a dated core sample.

Whether the bioherms are still growing however is an open question, and one the scientists are hoping to answer with further study. "Certainly, the Halimeda is a living veneer that sits on top of the bioherm," McNeil said. "We know that at times, the Halimeda is growing and at other times it's not."

Australia's Great Barrier Reef has consistently been in the news in 2016 after northern parts of the reef were hit hard by the ongoing global coral bleaching event, the full impact of which is still emerging.

Halimedabioherms are also important to the health of the reef, but the effect of global warming, and in particular, the impact of ocean acidification caused by higher water temperatures, is little understood.

"There is research currently emerging that the Halimeda may be susceptible to ocean acidification because it does have that calcium carbonate skeleton," she said. "Any organism in the reef, including coral or shellfish, which has a calcium carbonate skeleton, can potentially be affected by ocean acidification.

"Increasing ocean acidification may affect its ability to build that skeleton."

For McNeil, the team's research highlights just how little we know about the ocean.

For now, it's not even clear how other living things in the sea use the giant structures, which form part of the inter-reef habitat. They clearly provide refuge and a place for the dispersal of fish and prawn larvae, but exactly how they are used, from the tops of the mounds to the deep hollows in the centre of the rings, is something McNeil plans to study further.

"The surface areas and coral reef where we can go and visit are reasonably well studied, but there are vast expanses of ocean that are hidden away because of the difficulty in accessing them and because of the depth," she said.

"There's just so much we don't know about the sea floor and all the biology that uses it as habitat."

The study was published in the scientific journal, Coral Reefs.