The iconic Great Barrier Reef is hiding something.
Researchers have created the first detailed maps of the ocean floor behind the Great Barrier Reef and discovered a field of giant ring-shaped mounds of limestone, according to research published in the journal Coral Reefs.
There's essentially an entire second reef hiding behind the Great Barrier Reef.
"We've known about these geological structures in the northern Great Barrier Reef since the 1970s and '80s, but never before has the true nature of their shape, size and vast scale been revealed," Robin Beaman, a scientist who worked on the research, said in a statement.
The mounds are between 200 and 300 meters across, and some stand about 10 meters tall. They're called Halimeda bioherms. They form when green algae called Halimeda die and form flakes of limestone. The limestone accumulates over time into mounds.
Researchers created the maps by using planes equipped with a kind of laser radar. The field of mounds spans much farther than researchers previously believed.
"We've now mapped over 6,000 square kilometers," lead author Mardi McNeil said in the statement. "That's three times the previously estimated size, spanning from the Torres Strait to just north of Port Douglas."
Researchers say that the mounds might be in danger. Halimeda algae may be harmed by acidification of the oceans, caused by increasing carbon dioxide emissions. In the future, they hope to explore the area further using methods including robotic underwater vehicles to learn more about the structures and the ocean.