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

Unlike “black holes” that attract and swallow the surrounding space matter, a “gravitational hole” pushes the surrounding material away due to the lack of gravitational force. When it forms in the ocean, a gravity hole pushes the water out and creates air pockets where the water should be, lowering the sea level. Take the example of the world’s largest and deepest gravity well discovered in the Indian Ocean. The hole caused sea levels to drop 348 feet (106 meters), perplexing geologists for decades, until 2023, when some scientists offered a potential explanation in a study published in the journal Geophysical Research Letters. The mystery lies in the sunken ancient Indian sea, CNN reported.

This gravity hole, called the Indian Ocean Geoid Low (IOGL), is located about 1,200 kilometers (750 mi) southwest of Kanyakumari, the southernmost tip of the Indian subcontinent. It is a 1.2 million square mile (3 million square kilometer) circular depression lurking in the ocean waters. Compared to the surroundings, gravity is weaker in this area. “It’s by far the largest minimum in the geoid and it hasn’t been properly explained,” said study co-author Attreyee Ghosh, a geophysicist and associate professor at the Indian Institute of Science’s Center for Earth Sciences.

Initially, Dutch geophysicist Felix Andries Vening Meinesz discovered the hole in 1948 during a gravity survey from a ship. Felix invented a device called the “Golden Calf” to measure gravity on the seas, according to Big Think. Since then, researchers have been trying to explain the existence of this ocean chasm. “The origin of this geoid anomaly has been mysterious. Various theories have been put forward to explain this negative geoid anomaly,” the researchers wrote in the study. In 2023, some scientists at the Indian Institute of Science in Bengaluru claimed to have 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 the depths of 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 what most people believe, the Earth is not a perfect sphere. “The Earth is basically a lumpy potato,” she said, “so technically it’s not a sphere, but what we call an ellipsoid, because as the planet rotates, the middle part bulges out.”

Furthermore, the Earth is not uniform even in its density and properties. Some areas are denser than others, which affects the surface of the Earth and its gravity, she explained, adding: “If you pour water on the surface of the Earth, the surface that the water occupies is called the geoid – and it’s controlled by these differences in the density of material inside the planet because they attract the surface in very different ways depending on how much mass is underneath. Live Science described this geometric anomaly behind the geoid plain as: “The low is the result of our surprisingly delicate planet flattening at the poles, bulging at the equator, and undulating between lumps and bumps across its surface.

Ghosh and her fellow researchers took the whole story back to 140 million years ago. At the time, she said, “the continents and oceans were in very different places, and the density structure was also very different.” Starting from that time period, the team created 19 simulation models up to the present day that recreated the tectonic history and behavior of magma inside the mantle. In six of the models, a geoid low 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, which they believed were responsible for the formation of the “gravity hole,” Ghosh said. They further suggested that these plumes formed when the 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 the ocean disappeared and the gap with Asia closed,” she explained. According to the team, as the oceanic plate squeezed inside the mantle, it could have spurred cloud formation, bringing low-density material closer to 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 supercontinent Gondwana and collided with the Eurasian plate. This collision eventually resulted in the creation of the Himalayas, but before that happened the Indian plate overtopped the Tethys plate and pushed it under the Indian plate.

It was pushed into the mantle, which is today’s location near East Africa. Finally, about 20 million years ago, the subducting Tethyan plates displaced the trapped magma of the African blob, leading to the formation of the plumes. “These plumes, along with the mantle structure near the geoid low, are responsible for the formation of this negative geoid anomaly,” the researchers wrote in the study.

Discussing whether the gravity hole will stay or shift or disappear, Ghosh told CNN, “It all depends on how these mass anomalies move around on Earth. It is possible that it persists for a very long time. But it could also be that the plate motions will act in such a way that they disappear a few hundred million years into the future.”