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

Unlike “black holes,” which pull in and swallow surrounding cosmic matter, a “gravity hole” pushes surrounding material away due to the lack of gravity. When formed in an ocean, a gravity hole pushes water away and creates air pockets where water should have been, causing sea levels to drop. Take the example of the world’s largest and deepest gravity hole, discovered in the Indian Ocean. The hole caused sea levels to drop by 348 feet (106 meters) and puzzled geologists for decades, until 2023, when some researchers offered a possible explanation in a study published in the journal Geophysical Research Letters. The mystery lies in a submerged ancient Indian sea, CNN reported.

The gravity well, also called the Indian Ocean geoid low (IOGL), is located about 1,200 kilometers (750 miles) southwest of Kanyakumari, the southernmost tip of the Indian subcontinent. It is a 1.2 million square mile (3 million square kilometer) circular depression that lurks in the ocean waters. Compared to its surroundings, the gravity well in this area is weaker. “It is by far the largest depression in the geoid and it is not well explained,” said study co-author Attreyee Ghosh, a geophysicist and associate professor at the Indian Institute of Science’s Centre for Earth Sciences.

Initially, a Dutch geophysicist, Felix Andries Vening Meinesz, discovered the hole in 1948 during a gravity measurement from a ship. Felix had invented a device called the “Golden Calf” to measure gravity at sea, according to Big Think. Since then, researchers have been trying to explain the existence of this oceanic abyss. “The origin of this geoid layer is puzzling. Several theories have been put forward to explain this negative geoid anomaly,” the researchers wrote in the study. In 2023, researchers from the Indian Institute of Science in Bengaluru announced that they had found an explanation. Their hypothesis is that the hole was formed as a result of an ancient ocean that no longer exists. They believe that plumes of magma rising from deep within the planet are responsible for the existence of this gravity hole.

To lay the groundwork for understanding, Ghosh explained that the secret lies in the geometry of the Earth. Contrary to popular belief, the Earth is not a perfect sphere. “The Earth is basically a lumpy potato,” she said, “so technically it’s not a sphere, it’s what we call an ellipsoid, because the middle part bulges outward as the planet rotates.”

Furthermore, Earth isn’t even uniform in its density and properties. Some areas are denser than others, which affects the Earth’s surface and gravity, she explained, adding: “If you pour water on the Earth’s surface, the level that the water takes is called a geoid — and that’s controlled by these density differences in the material 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 layer as: “The layer is a consequence of our surprisingly soft planet, which flattens at the poles, bulges at the equator, and undulates between bumps and bulges across the surface.”

Ghosh and her fellow researchers took the whole story back to 140 million years ago. Back then, she said, “the continents and the oceans were in very different places, and the density structure was also very different.” From that timescale, the team ran 19 simulation models all the way to the present day, reconstructing the tectonic history and the behavior of magma in the mantle. In six of the models, a geoid layer similar to that in the Indian Ocean formed.

In each of these six models, they observed the presence of plumes of magma around the geoid layer, which they believed were responsible for the formation of the “gravity hole,” Ghosh said. They further suggested that these plumes formed when an ancient Indian Ocean disappeared millions of years ago.

“India was in a very different place 140 million years ago, 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, the compression of the oceanic plate into the mantle could have caused the plumes to form, bringing low-density material closer to the Earth’s surface, reducing the region’s mass and weakening its gravity. More than 100 million years ago, the Indian plate broke off from the supercontinent Gondwana and collided with the Eurasian plate. This collision eventually led to the formation of the Himalayas, but before that happened, the Indian plate crossed over the Tethys plate and pushed it under the Indian plate.

It was pushed into the mantle, which is now located near East Africa. Eventually, about 20 million years ago, the subducting Tethyan plates displaced the trapped magma from the African blob, leading to the formation of the plumes. “These plumes, together with the mantle structure near the geoid layer, are responsible for the formation of this negative geoid anomaly,” the researchers wrote in the study.

Discussing whether the gravity hole will remain, shift or disappear, Ghosh told CNN: “It all depends on how these mass anomalies in the Earth move. It could be that it will remain for a very long time. But it could also be that the plate movements will work in such a way that it will disappear, a few hundred million years into the future.”