What lies off Australia’s Great Barrier Reef, in the Coral Sea? The region was mostly unexplored and uncharted until a recent expedition searched its dark waters, uncovering an abundance of life, weird geologic features and spectacular deep corals. The deepest forays reached down more than a mile.

“It blew our minds,” said chief scientist Robin Beaman of James Cook University. “We’ve gone from literally knowing nothing to knowing a lot.”

An earlier leg of the expedition found a gelatinous creature whose length was estimated at 150 feet — potentially the world’s longest example of oceanic life. The Coral Sea lies northeast of Australia, adjacent to the Great Barrier Reef, and includes a vast protected zone.

The region’s thriving population of chambered nautili was a big surprise, Beaman said. The creature is a living fossil whose ancestors go back a half-billion years — to the early days of complex life on the planet. The logarithmic spiral of the animal’s pearly shell echoes the curved arms of distant galaxies, and its beauty has turned the nautilus into a prized commodity, with some of its populations decimated by hunting and trapping.

“They were everywhere, bobbling around,” Beaman said. “They’re really beautiful animals to look at.” The expedition explored a protected zone hundreds of miles off the coast, and he said the remoteness helped safeguard the creatures.

In all, the expedition mapped more than 13,000 square miles of seabed — an area bigger than Massachusetts — and transformed unknown waters into carefully evaluated realms of wilderness. It was organized by the Schmidt Ocean Institute, founded by Eric Schmidt, the former chairman of Google, and his wife, Wendy. Its centerpiece was a ship that could map the remote seabed with beams of sound and deploy robots to capture close-up images.

Beaman said another surprise was the complexity of the deep seabed around the region’s 30 large coral atolls and banks. The expedition found submarine canyons, dune fields, submerged reefs and massive landslides.

“We had no idea what these steep flanks looked like,” he said. “We were astounded. There’s lots of evidence of undersea landslides, some probably millions of years old.”

The team also found the deepest living hard corals in eastern Australian waters and identified as many as 10 new species of fish, snails and sponges.

Beaman noted that the process of discovery from the expedition had only begun; widening circles of scientists and students were now starting to examine the new seabed maps and the video recordings from the robot’s abyssal dives. “The imagery will be worked on for years,” he said. “It was an amazing experience.”