The mysterious ‘gravity hole’ in the Indian Ocean that has baffled scientists for years

Unlike “black holes” that attract and swallow surrounding cosmic matter, a “gravity hole” repels surrounding matter due to the lack of gravitational force. When formed in an ocean, a gravity hole pushes water away and creates pockets of air where there should have been water, lowering sea levels. Consider the world’s largest and deepest gravity hole, discovered in the Indian Ocean. Causing a 350-foot drop in sea levels, the hole puzzled geologists for decades, until 2023, when some researchers offered a potential explanation for it in a study published in the journal Geophysical Research Letters. The mystery lies in an ancient sunken Indian sea, CNN reported.

Called the Indian Ocean Geoid Depression, the gravity hole is located about 1,200 kilometers southwest of Kanyakumari, the southernmost tip of the Indian subcontinent. It is a 3-million-square-kilometer circular depression lurking in the ocean waters. Compared to its surroundings, gravity is weaker in this area. “This is by far the largest depression in the geoid, and it has not been properly explained,” said study co-author Attreyee Ghosh, a geophysicist and associate professor at the Centre for Earth Sciences at the Indian Institute of Science.

The hole was originally discovered in 1948 by a Dutch geophysicist, Felix Andries Vening Meinesz, during a gravity survey aboard a ship. Felix had invented a device called the “Golden Calf” to measure gravity at sea, according to Big Think. Since then, researchers have tried to explain the existence of this oceanic abyss. “The origin of this geoid depression is enigmatic. Different theories have been put forward to explain this negative geoid anomaly,” the researchers write in the study. In 2023, researchers from the Indian Institute of Science in Bengaluru claimed to have found an explanation. Their hypothesis is that the hole formed as a result of an ancient ocean that no longer exists. They believe that plumes of magma rising from the depths of the planet are responsible for the existence of this gravity hole.

To lay the foundation for understanding, Ghosh explained that the secret lies in the geometry of the Earth. Contrary to what most people think, the Earth is not a perfect sphere. “The Earth is actually a bumpy potato,” she said. “So technically, it’s not a sphere, but what we call an ellipsoid, because as the planet rotates, the central part bulges outward.”

Additionally, Earth is not uniform in its density and properties. Some areas are denser than others, which affects Earth’s surface and its gravity, she explained, adding, “If you pour water onto the surface of Earth, the level that water takes is called a geoid — and it’s controlled by these density differences in the materials inside the planet because they pull on the surface in very different ways depending on how much mass is underneath.” Live Science described this geometric anomaly behind the geoid depression as: “The depression is a consequence of our surprisingly soft planet, which flattens at the poles, bulges at the equator, and undulates between bumps and humps on its surface.”

Ghosh and her fellow researchers stretched the entire history back 140 million years. She said that at that time, “the continents and the oceans were in very different places, and the density structure was also very different.” From that time scale, the team ran 19 simulation models back to the present day, recreating the tectonic history and behavior of magma inside the mantle. In six of these models, a geoid depression similar to the one in the Indian Ocean formed.

In each of these six models, they observed the presence of magma plumes around the geoid depression, which are thought to be responsible for the formation of the “gravity hole,” Ghosh explained. They also suggested that these plumes formed when an ancient Indian Ocean disappeared millions of years ago.

“140 million years ago, India was in a very different place, and there was an ocean between the Indian plate and Asia. India started moving north, and as it did, the ocean disappeared and the gap with Asia closed,” she explained. According to the team, as the oceanic plate crashed into the mantle, it could have caused plumes to form, bringing low-density material closer to the Earth’s surface, reducing the mass of the region and weakening gravity. More than 100 million years ago, the Indian plate broke away from the Gondwana supercontinent and crashed into the Eurasian plate. This collision eventually formed the Himalayas, but before that, the Indian plate broke through the Tethys plate, pushing it under the Indian plate.

The magma was pushed into the Earth’s mantle, where it is today near East Africa. Eventually, about 20 million years ago, the sinking Tethyan plates displaced the trapped magma in the African mass, leading to the formation of the plumes. “These plumes, along with the mantle structure near the geoid depression, are responsible for the formation of this negative geoid anomaly,” the researchers wrote in the study.

“It all depends on how these mass anomalies move on Earth. It could be that it persists for a very long time. But it could also be that the plate motions act in such a way that it disappears in a few hundred million years,” Ghosh told CNN.