The ocean is vast, deep, and unexplored.

When Malaysia Airlines Flight 370 disappeared three years ago this week, the search brought the ocean’s vastness into sharp relief. This is how deep and dark it is three miles down. This is how unlikely you are to spot a downed airliner in 120,000 square nautical miles of open ocean. This is how much we know about the ocean floor—less than we know about the surface of Mars.

As the search dragged on and sonar swept the Indian Ocean, data also piled up. The surveyors found volcanoes and valleys and scars on the sea floor from tectonic plates pulling apart. This arc of ocean, delineated by calculations based on the plane’s last satellite pings, has by chance become one of the most thoroughly mapped regions of the ocean floor.

Scientists with Geoscience Australia, which provided technical advice and support for the search, recently published some of their findings. All of the underwater survey data will be released later this year.

The search area for MH370 is a remote part of the Indian Ocean a couple thousand miles west of Australia. “Geoscience Australia didn’t have any specific scientific interest in the area,” Kim Picard, one of the agency’s scientists, said in an email. But it was obvious that a high-resolution map of 100,000 square miles of ocean floor would have considerable scientific value in addition to aiding in the search for MH370.

The search area that was mapped using multibeam sonar. Geoscience Australia

Most of what we know about the ocean floor actually comes from gravity-sensing satellites. Gravity is a function of mass, so the areas where Earth’s crust is thicker—like where there is an underwater mountain—will exert ever so slightly more pull on the water. This creates small but detectable bumps on the surface of the water. Using this technique, scientists have mapped the contours of the ocean floor to a resolution of about 2 square miles per pixel. That’s remarkable for data from satellites orbiting many miles above Earth, but not terribly useful when you’re actually down on the ocean floor looking for a missing plane.

Multibeam sonar, on the other hand, can resolve features about the size of a soccer field. The first phase of the search and recovery operations was simply mapping the ocean floor using this sonar, which measures the time it takes for a sound wave to bounce back from the seabed. The sonar alone would not have found a plane though. “It is still too coarse to pick out an individual debris or rock, nor could it distinguish rock from metal,” said Picard. The sonar map was created to aid the second phase of operations, where underwater drones scanned the ocean floor in more detail.

A 3D model of the Broken Ridge, mapped using multibeam sonar. Geoscience Australia

But even the sonar data was such a leap forward. Under that smooth blanket of water, the ocean floor is dynamic and dramatic. Among the features Picard and her team found were escarpments suddenly rising thousands of feet into the water, features reshaped by underwater landslides, and deep fractures from tectonic plates edging apart.

The water in the search area was three miles deep. Even for a marine geoscientist, seeing such a deep and remote area of the ocean in such detail was unusual. “We normally work on smaller surveys that mainly occur on the continental shelf and slope, so it is much shallower water and much closer to home,” said Picard. Only about 10 to 15 percent of the ocean has been mapped with technologies such as multibeam sonar.

The search for MH370 is now officially suspended. Although debris from the plane has turned up in Africa and islands of the Indian Ocean, nothing has been found in the search area. This remote part of the Indian Ocean is one of the best-mapped areas of the underwater world, but still, we can’t tell whether a lost airplane is there or not.