Science answers 75-year riddle of the Indian Ocean

The Indian Ocean hides one of Earth’s strangest secrets: a vast region where gravity is so weak, the sea there drops more than 100 meters below its surroundings. For 75 years, scientists were baffled by this phenomenon known as the Indian Ocean Geoid Low. Why does gravity vary so much here, and what invisible force is at work beneath the waves?

Now, after decades of speculation, researchers have an answer grounded deep within our planet’s mantle. Using supercomputer simulations, scientists linked this bizarre gravity hole to unusual plumes of hot, lightweight rock rising from the deep, a pulse of motion that began more than 140 million years ago.

An Unexpected Dip in the Depths

Seen from space, Earth’s oceans appear smooth. But remove the tides and currents, and you’d find the surface undulates gently, shaped by gravity’s subtle pull. In most places, seawater clings close to the geoid Earth’s level surface.

But over a patch 1.2 million square miles wide in the Indian Ocean, called the IOGL, the water sinks dramatically lower than anywhere else on the planet. Discovered by geophysicist Felix Andries Vening Meinesz in 1948, the IOGL has mystified generations.

Over the years, researchers floated theories from sunken tectonic plates to hidden mantle plumes, yet none explained its size or depth. Even state-of-the-art gravity satellites couldn’t pierce the mystery until now.

Did you know?
The Indian Ocean Geoid Low is the deepest gravity pit on Earth, with the ocean surface dipping by up to 106 meters, enough to submerge a 30-story building.

Unveiling the Mantle’s Secret

A breakthrough arrived when Prof. Attreyee Ghosh of the Indian Institute of Science and her team, working with German geoscientists, recreated Earth’s interior using advanced numerical models and seismic tomography data.

Their approach traced the motions of continents, ancient oceans, and deep mantle dynamics back 140 million years. Their supercomputer tests revealed six scenarios matching today’s geoid low, each driven by the rise of low-density magma beneath the Indian Ocean’s crust.

Instead of a single plume directly under the IOGL, they found evidence for hot material from the African superplume, deflected outward and pooling beneath the Indian Ocean. This upwelling replaced heavier rock with lighter, warmer material from below, creating a vast mass deficit and the gravity dip we observe today.

Ancient Oceans, Modern Mysteries

This story began as India’s landmass sailed north from Gondwana, closing an ocean gap with Asia. The ancient oceanic plate plunged into Earth’s mantle, spurring the upwelling of hot rock that still shapes the geoid today. The gravity low, calculations reveal, most likely formed about 20 million years ago and may persist far into the future.

Plate tectonics, mantle convection, and the planet’s slow internal churn have sculpted a region that makes the Earth, once thought to be a tidy sphere, look more like a lumpy potato. As continents moved, the legacy of ancient oceans imprinted itself on forces beneath our feet and, in turn, altered the shape of the sea far above.

ALSO READ | What Secrets Does 1.5M-Year Ice Hold About Climate?

Frontiers and Future Questions

Not every detail is settled: Some scientists note the models miss volcanic features, such as the Réunion plume and the Deccan Traps eruption. The simulations also show only an 80% match between observed and predicted gravity data.

Yet despite those caveats, most agree this work is the closest we’ve come to solving the IOGL puzzle, linking the gravity depression to mantle mass anomalies that have shifted over millions of years.

Ongoing research will refine these models and probe the Earth’s interior with ever greater precision. For now, the Indian Ocean gravity hole stands as a reminder: hidden deep within the planet, ancient geologic processes still shape the face of the world’s oceans.