There's an enormous gravity hole in the middle of the Indian Ocean. Scientists say they've finally worked out what's causing it.

• There's a gravity hole in the Indian Ocean, where ocean levels are about 300 feet lower than surrounding areas.
• Local gravity is slightly lower there, and this has puzzled scientists. 
• The gravity hole may have been caused by an ancient ocean bed that sank millions of years ago.

There's a huge gravity hole in the middle of the Indian Ocean that has stumped scientists for decades.

The million-square-mile anomaly isn't a physical hole, but an area of the ocean where Earth's gravity is lower than average.

Scientists studying the "hole" have long thought there must be something underneath it that is causing the strange effect.

But a new study suggests researchers should have been looking around, not under, the gravity hole to solve the mystery of how it formed. 

They claim that plumes of molten rock rising from the remnants of an ancient ocean bed could be to blame. 

A million-square-mile gravity hole that is lowering the oceans

Gravity varies very slightly over the surface of the globe.

Most of these variations can be easily explained. A dense continent, for instance, may have more mass and therefore slightly higher gravity than, say, a zone where the crust is thinner.

But scientists have struggled to explain the gravity hole in the Indian Ocean, known as the Indian Ocean geoid low.

The difference in gravity is not huge. You certainly wouldn't be able to notice it if you were standing right in the middle of the anomaly, Bernhart Steinberger, a geodynamics researcher at GFZ German Research Centre of Geosciences, told Insider.

But it is significant enough that ocean levels over the 1.1-million-square-mile patch are about 300 feet lower than in the surrounding oceans. 

"I think what people generally assume is that there must be something low density underneath that's causing that," said Steinberger.

"But in that paper, they have actually a different theory," he said.

To understand the anomaly, scientists had to look around the hole

To understand what's causing the hole, geophysicist Attreyee Ghosh and doctoral student Debanjan Pal at the Indian Institute of Science in Bangalore asked a computer to simulate what may have happened.

They plotted 19 different scenarios looking at how tectonic plates may have moved around the hole over the past 140 million years.

Their study, published in Geophysical Research Letters in May, found that only a few scenarios could account for the gravity hole, and in none of these models was the low gravity caused by what was directly underneath it.

Instead, they found the hole was likely caused by plumes of low-density magma. 

"It's something you could have thought of before, you just wouldn't think of it because you tend to think there must be something underneath," said Steinberger, who was not involved in the study.

"You have like a negative cutout," he said.

It goes back 120 million years ago

The most likely explanation for the gravity hole goes back to the separation of Gondwana, the supercontinent at the origin of Africa, Australia, and India, about 120 million years ago, according to the study.

As India separated from Africa and went to smash into the European plate, the ocean that used to be there, called Tethys, was split apart and squished between the continental plates.

Some tiny parts of the plate are still present in the Mediterranean, but most of that plate is still slowly melting back into the deep Earth's interior around Eastern Africa. As the dense mantle melts away, it creates plumes of low-density magma, contributing to the low gravity area.

At the same time, surrounding masses like the Tibetan plateau, create a gravity high, amplifying the effect, Steinberger said. 

Looking forward, surveys of the oceans will have to confirm that these plumes exist in real life, not just on computers, Himangshu Paul at the National Geophysical Research Institute in India, told New Scientist.