There’s a big hole in the Indian Ocean. It’s nothing you can actually see. And the ocean itself isn’t especially deep. Instead, it’s a hole in Earth’s gravitational field—the weakest pull across the entire planet.

The “hole” was discovered in 1948. It’s centered about 750 miles off the southwestern coast of India. It covers more than a million square miles—more than a third the area of the Lower 48 states. Gravity there is so weak that surrounding regions of the ocean pull water away from it. As a result, sea level above the hole is about 350 feet lower than the global average.

In 2023, using computer models of the motions of the plates that make up Earth’s crust, scientists suggested the hole may be the remnant of another ocean—the Tethys Ocean. It vanished tens of millions of years ago.

The ocean was wedged between two “super”-continents—slabs that held most of the world’s total land area. But the motions of the plates pulled apart one of the continents. That pushed the plate that held the Tethys Ocean deep into the mantle—the layer below the crust.

The ocean floor reached its deepest point below the surface about 20 million years ago. It pushed away dense blobs of rock, allowing lighter rock to bubble up from below. The lighter rock exerts a weaker pull than the rocks around it.

Scientists still need to confirm that scenario—a possible explanation for a giant “hole” at the bottom of the Indian Ocean.