Earth's biggest gravity hole, where the ocean surface dips 106 meters below where it should be. An ancient tectonic plate sank into the mantle here 30 million years ago, leaving a mass deficit. If you placed a ball on the water, it would roll toward the edges of this invisible depression.
A 3 million square kilometer anomaly
The Indian Ocean Geoid Low (IOGL) is the planet's largest and most pronounced gravitational anomaly. Spanning an area of about 3 million square kilometers just south of the Indian peninsula, this feature is an enormous depression in the Earth's geoid. The geoid is the theoretical surface of the oceans if they were influenced only by gravity and the planet's rotation, and it is not a perfect sphere. Due to uneven mass distribution within the Earth's mantle, the geoid has lumps and dips. The IOGL is the most significant of these dips, with a sea level that is 106 meters (about 348 feet) lower than the global average.
This "gravity hole" was first discovered in 1948 by Dutch geophysicist Felix Andries Vening Meinesz during a ship-based gravity survey. But its cause remained a puzzle for decades. Modern satellite missions, such as GRACE and GOCE, have since mapped the feature in detail, confirming its immense scale. The gravitational pull in the region is weaker than normal, a direct consequence of a mass deficit deep beneath the ocean floor. This deficit is not visible on the seafloor itself, which appears normal; the source lies much deeper in the mantle.
Sinking plates and rising plumes
The leading explanation for the IOGL involves events that began over 140 million years ago. Researchers at the Indian Institute of Science, Debanjan Pal and Attreyee Ghosh, developed computer simulations to model the region's deep history. Their models suggest the anomaly is the result of ancient tectonic plate movements interacting with deep mantle structures.
The story begins with the northward drift of the Indian plate after it broke from the supercontinent Gondwana. As it moved, it pushed the seafloor of the ancient Tethys Ocean down into the mantle—a process called subduction. These cold, dense slabs of oceanic crust, known as the Tethyan slabs, sank deep into the lower mantle over millions of years.
Around 20 to 30 million years ago, these sinking slabs reached the base of the mantle and disturbed a hot, dense structure known as the African Large Low-Shear-Velocity Province, or the "African blob". This disturbance triggered plumes of less-dense, hot mantle material to rise upwards. These buoyant plumes spread out beneath the lithosphere of the Indian Ocean, creating a large region of lower-density material stretching from a depth of about 300 to 900 kilometers. This mass deficit causes the weaker gravity and the 106-meter depression in the geoid that exists today. The simulations show the feature likely took its current shape around 20 million years ago.
💡Fun Facts
The geoid low is an invisible feature; the 106-meter drop is only relative to the global average sea level and cannot be seen with the naked eye.
The area of the gravity hole is roughly 3 million square kilometers, nearly the size of India itself.
Computer models needed to simulate 140 million years of tectonic plate movement to explain how the anomaly formed.
The sunken slab of crust responsible for the anomaly is a remnant of the ancient Tethys Ocean, which once separated the supercontinents of Laurasia and Gondwana.
Free, though requires chartering a vessel capable of reaching remote international waters.
Accessibility
The feature is located in the open ocean. It is an invisible depression in the planet's gravity field and has no physical or visual presence on the water's surface.
The Indian Ocean Geoid LowIndian Ocean, International Waters