There's a massive "gravity hole" in the Indian Ocean where the pull of gravity is less than everywhere else on Earth and the sea level is is 100 meters lower than the planetary average. WTF, geologists have said. But now they know the likely source of this strange phenomenon: plumes of molten lava spewing from beneath Africa. The behavior of this magma and variances in the mass of the Earth's crust and mantle affect the planet's gravity on the surface.
"The Earth is basically a lumpy potato," said study coauthor Attreyee Ghosh, a geophysicist and associate professor at the Centre for Earth Sciences of the Indian Institute of Science. "So technically it's not a sphere, but what we call an ellipsoid, because as the planet rotates the middle part bulges outward."
Our planet is not homogeneous in its density and its properties, with some areas being more dense than others — that affects Earth's surface and its gravity, Ghosh added. "If you pour water on the surface of the Earth, the level that the water takes is called a geoid — and that is controlled by these density differences in the material inside the planet, because they attract the surface in very different ways depending on how much mass there is underneath," she said.
The "gravity hole" in the Indian Ocean — officially called the Indian Ocean geoid low — is the lowest point in that geoid and its biggest gravitational anomaly, forming a circular depression that starts just off India's southern tip and covers about 1.2 million square miles (3 million square kilometers)[…]
"It is by far the biggest low in the geoid, and it hasn't been explained properly," Ghosh said.