You can think of Earth as a perfectly shaped sphere, especially if you look at the planet’s profile from outside its atmosphere. In every image we see of the planet, it looks like a perfect circle. However, this is not entirely true. In fact, there are areas that are much flatter than others, and there is even a “gravity hole” in the middle of the Indian Ocean, according to new research.

Of course, that’s not what you might think of when you hear the term “hole”. The water is not flowing out of the ocean and it is not a mysterious blue hole. Instead, a “hole” is a place in Earth’s geoid where gravity is below average, causing sea level to be below the global average. So what exactly is the cause of this “gravity hole”? Researchers may have finally found the answer.

According to a new study published Geophysical Research Letters, this low gravity is most likely due to magma plumes. The plumes may have helped Earth take its current shape more than 20 million years ago, when the current geoid (an imaginary representation of what sea level looks like on Earth) was formed.

The plumes have probably continued to move for the last 20 million years, and when they stop, the low shear geoid we see in the middle of the Indian Ocean is likely to shift, causing the gravitational hole to dissipate. It’s an intriguing discovery that could help us understand the planet’s geoid a little better, especially as researchers delve deeper into it.

While it’s impossible to see the difference on the ocean’s surface, it’s there and the researchers say the plumes responsible for the gravity hole will likely stay put for a few million more years. By then, we hope we can learn more about our geoid and the precise causes of the various gravitational forces on our planet.