Voyage uncovers Indian Ocean mysteries

In a remote part of the Indian Ocean, gargantuan underwater mountains and mysterious animals thrive in an underwater labyrinth, and Melbourne scientists have gone to the abyss to find them. 

The Museums Victoria Research Institute has led the mammoth task of mapping out thousands of kilometres of ocean floor in new marine parks around the Cocos Keeling Islands and Christmas Island. 

The voyage is the second stretch for a research project that began in 2021, with the findings to inform how Parks Australia manages and protects the parks. 

The detailed mapping is a first for the Cocos, while both parks were previously unexplored by biologists, according to the institute's chief scientist Dr Tim O'Hara.

From the CSIRO's RV Investigator, researchers have mapped the structure of the ocean floor, deploying multi-beam sonar that use sound signals to detect the seabed, and sending cameras down to 5500 metres. 

In an operations rooms on the ship, Dr O'Hara has marvelled at the underwater landscape, with equipment picking up on five-kilometre high mountains and revealing an array of fish hovering over the summits of seamounts. 

"The surface of the water just looks like the surface of the water, so it's a bit of a mind game to think that down on the bottom of the sea, (there's) all of this amazing landscape with huge volcanoes, mountains and ridges, " Dr O'Hara told AAP.

"You've got to remember, there it's completely dark ... so as the cameras come down and we're shining a light in front of us, you can see the fish are startled."

The team has uncovered ancient sea mountains, volcanic cones, canyons and ridges in the Cocos Keeling Islands marine park.

They have also produced detailed three-dimensional images of the mountain underneath the islands themselves, which shows them as twin peaks of a seamount that rises nearly 5km from the sea floor. 

The team set off on their 11,000-kilometre voyage from Darwin at the end of September, and will return to port near Perth on Thursday after 35 days of exploration.

With them, they'll bring a haul of previously undiscovered animal species after conducting up to three ocean trawls a day, each of which could take up to four hours. 

Mr O'Hara estimates between one-quarter and one-third of the animals they have brought up might be previously undiscovered species, with each trawl bringing up to 10 different specimens. 

"We've got a winch with eight kilometres of wire on it ... and we generally attach a small net or what's called a dredge, which is like a metal box, and we lower it carefully down to the sea floor," Dr O'Hara said. 

The new species will be sent off to experts, with Dr O'Hara anticipating it will take them a couple of years to describe them. 

Among the team's finds are previously unknown tripod fish and deep-sea eels, including a blind eel collected about 5km below the surface and covered in loose, transparent, gelatinous skin.

The voyage has also let scientists get eyes on rarely seen fish, including the sharp-toothed hermaphrodite highfin lizardfish, and batfishes that walk on the sea floor on their armlike fins. 

The team's findings were essential to inform authorities about life within the marine parks, and help answer the question of how life moved through Indonesia between the Pacific and Indian oceans, Dr O'Hara said.