There might be an answer to why there is a gravitational hole in Earth's gravitational field.

The gravitational field at the point of the Indian Ocean is weaker than in other places on Earth. The existence of that gravitational hole is proved. But the reason why that gravitational hole is formed is unknown. The reason why the gravitational field on the Indian Ocean is weaker might be a simple thing. 

There is a possible lighter rock in the mantle, or there is less material at that point. In some wildest theories, there are bubbles in magma that turn below the bottom of the Indian Ocean are less massive than in other places on Earth. The lighter material at that point should make the hole in gravitation. 

"Geoid undulation in false color. (International Centre for Global Earth Models/Wikimedia, CC BY 4.0)

The impact of the ancient planet. In the time when Earth was young. Cause asymmetry of Earth's gravitational field, because there still is the debris of that impact. But the hole in a gravitational field is harder to explain than regular-looking asymmetry. 

Could the reason for that gravitational hole be the gravitational geyser that sends gravitational waves around the gravitational pool? That puts gravitational waves start to travel away from the point. There that gravitational pike crosses the weaker gravitational field.

In some wildest theories, gravitational waves or gravitational pike causes that hole. That pushes the other parts of Earth's gravitational field away. The idea is that there is some kind of gravitational tornado or gravitational geyser that pushes other gravitational fields away. The higher energy gravitational geyser sends gravitational waves around it. And if the gravitational waves have the same wavelength but the energy level is higher. That thing causes a situation that the stronger gravitational field pushes the weaker gravitational field away. 

The confirmation of the reason why that gravitational hole exists is not given. So that is one of the most interesting things in natural sciences. Gravitation is one of the fundamental interactions. So gravitation should affect in the same way to all components in the gravitational field. Particles form the quantum along with electromagnetic and gravitational fields. 

The thing that there is some kind of hole in the gravitational field means that there is some kind of segment that is not part of the quantum entirety in the system. And that is one of the things that makes this kind of gravitational research interesting. 

Could there hole in black hole's gravitational field?

If we want to transfer this kind of hole in extremely strong gravitational fields there is a possibility that at least in the poles of low mass black holes gravitational field is the hole. The idea is this the hole in that extreme gravitational field forms when a black hole spins very fast. That spin makes the black hole turn flat. In that model, there is less material at the point of the poles of singularity. 

So that thing should make it possible that the gravitational field of black holes is not homogenous. The force of the black hole's gravitational field is very strong. And in those force levels, even small changes in the gravitational or other force fields has a big effect. 

https://www.sciencealert.com/theres-a-giant-gravity-hole-in-the-indian-ocean-and-we-may-finally-know-why