McNeil's job was to map the extent and structure of bioherms, mounds of ancient calcified algae, scattered outside the Great Barrier Reef 60 to 100 feet below the surface. Scientists in the 80s had documented the bioherms using acoustic sound waves, but no one had re-visited them with modern technology. So McNeil used lidar data (a mapping technique that measures distance with lasers) collected by the Australian navy to create high-resolution 3D renderings of the ocean floor. But as soon as she started looking through the images she made, McNeil knew she'd found something big.
Scientists had previously thought the bioherms covered an area of about 800 square miles. McNeil's map showed a bioherm network covering over 2,300 square miles, triple the original area estimate. And it wasn't just the extent of the bioherms that was a surprise: it was their shapes. There were huge doughnut-shaped bioherms, 30 feet tall and 600 feet across, honeycombed sheets of bioherms with sharp ridges and crests, and undulating waves of bioherms spread across the ocean floor for miles and miles.
McNeil was on Skype with the other scientists on her team at the moment of discovery. "We were sharing a screen and just scrolling down the coastline, looking at it unfold, looking at these weird circular shapes, and we were all amazed," she says. "It just went on and on."
North-westerly view of bioherms in the Bligh Reef area off Cape York.
Depths are coloured red (shallow) to blue (deep), over a depth range of about 50 metres.
Bathymetry data copyright Australian Hydrographic Service.
The bioherm reef is believed to be about 8,000 years old, the same age as the coral of the Great Barrier Reef. Their mounds of ancient algae are a kind of geochemical archive, containing records of ocean temperature and acidity from the last Ice Age until now. The records can be used to understand how past ocean changes affected corals and other organisms, in order to predict how today's warming and acidifying oceans could affect future ecosystems.
The bioherm reef could be an important part of that future: because it's deeper than the Great Barrier Reef, the water around them is cooler and has a different chemical environment. McNeil says this kind of deep reef, called a mesophotic reef, has potential to provide refuge for sensitive species as the ocean warms.
But at the moment of her big discovery, McNeil wasn't thinking about the future. She was thinking about her scientific predecessors who mapped the bioherms in the 80s. "They did a really good job of mapping and interpreting the shape based on the technology they had at the time," she says, but they couldn't come close to what today's technology revealed. "If they could see this they would just be completely blown away."