The Indian Ocean has the lowest gravity anomaly on the planet, located south of Sri Lanka. This 'Indian Ocean geoid low' means the sea level here is over 100 metres lower than the global average, although the water looks calm. For years, scientists did not know why this region’s gravity is so different. New research combining satellite data, seismic imaging, and mantle modeling shows this strange feature comes from deep inside Earth. Movements of tectonic plates, sinking ancient ocean crust slabs, and hot material rising from the Earth’s mantle shaped the gravity low over tens of millions of years. The geoid is an invisible surface where Earth’s gravity is equal everywhere, close to average sea level. Most areas show little change, but the Indian Ocean geoid low is unusually big and deep. NASA’s satellites reveal the ocean crust here sits hundreds of metres lower than expected. This means the lack of mass is deep in the mantle, not just near the surface. The new 2023 study, “How the Indian Ocean Geoid Low Was Formed,” tracks back over 100 million years. It follows the Indian plate moving north, closing the Tethys Ocean and crashing into Asia. As the ocean vanished, slabs of ocean floor sank deep into the mantle. These sinking slabs disturbed other deep areas under Africa. This disturbance pushed a massive hot mantle region called the African Large Low Shear Velocity province. This caused hot plumes to rise beneath the Indian Ocean. These plumes spread under the crust, lowering the mantle’s density. Around 20 million years ago, this heating moved closer to the surface, deepening the gravity low. The lowest gravity does not line up with the hottest spots. Instead, it appears where several mantle influences mix. Warm upper mantle areas create a shallow low, deeper hot regions stretch it, and distant plumes shape its edges. Models show removing any one part weakens the match. This discovery shows that India’s plate motion and deep mantle processes together created the Indian Ocean’s unique gravity dip over millions of years.