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Ref. There is a ‘gravity hole’ in the Indian Ocean, and scientists now think they know why

According to CNN, the 'gravity hole' is an area of decreased gravity, compared with the surroundings. This article says that the lower gravity causes to sea level to locally fall (~100m). It seems to me that if there is less gravity at the 'hole' relative to the surroundings, the surrounding higher gravity would cause the sea level to rise up in a swelling over the hole, whilst pulling down the sea all around the hole. Just as the Moon's gravity pulls the seas towards it.

So why does the sea go down rather than up?

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    $\begingroup$ "Just as the Moon's gravity pulls the seas toward it" - note that there are two tidal bulges, one facing the Moon and one facing away from it. It doesn't matter whether the pulling force is "up" (for the near bulge) or "down" (for the far bulge), you get the bulge either way. A force up or down causes water to "pile up", so a lack of force logically results in a lower water level. $\endgroup$ Commented Jul 25, 2023 at 17:10

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This is because regions with higher gravity attract ocean water drawing it away from regions with lower gravity, leaving less water and lower sea level in the lower gravity regions.

Higher gravity regions can't "pull down" the water because water is almost incompressible and there is no place "down" for the water to go. The water just piles up over high gravity regions.

This reminds me of the Greenland sea level paradox: If all the ice in Greenland melted, the local sea level around Greenland would actually drop because the Greenland ice cap is so massive it gravitationally attracts water into the North Atlantic raising local sea level.

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    $\begingroup$ Ehh, water is "almost" incompressible. At around 4km depth, its roughly 1.8% compressed. en.wikipedia.org/wiki/Properties_of_water $\endgroup$
    – David S
    Commented Jul 25, 2023 at 16:04
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    $\begingroup$ Thanks @David-S, you make a valid point. I know water is slightly compressible but sloppily left out the "almost" in front of "incompressible" since the small compressibility wasn't important for my answer. I have now added the "almost". $\endgroup$ Commented Jul 26, 2023 at 14:01
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    $\begingroup$ It's more like "functionally incompressible". You really don't use water as if it is a compressible material. 1.8% after 4KM and pressures of 40 MPa. With 1 MPa = 145 psi, we're talking about almost 6,000 psi. $\endgroup$
    – Nelson
    Commented Jul 27, 2023 at 6:17
  • $\begingroup$ I do think compressibility does matter: 1.8% of 4000m is 72m ! $\endgroup$
    – summerrain
    Commented Mar 9 at 21:25
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Images to graphically describe what I believe David Bailey's answer is explaining.

Gravity doesn't just pull down. It also pulls sideways. When gravity is equal everywhere, the vertical and horizontal components cancel out, for a flat surface.

Equal gravity has a flat surface.

When gravity is unequal, there's less vertical pressure on the water, but there's also less force pulling inwards. So the high-gravity areas around the low-gravity area are able to pull the water sideways, away from the low-gravity area.

Unequal gravity has a not flat surface.

More images to graphically describe what I believe user253751'a answer is explaining.

Water follows an imaginary surface of equal gravitational potential. Imagine two different sources of gravity with different strengths. Both objects will have strong (red) gravity closer, and medium (orange) and light (yellow) gravity further out. But the radius at which the high gravity object pulls with a given strength is farther away from the center of the object.

Dense object has potential farther out.

If we move the objects close together, their gravitational fields merge.

Objects near each other mix their fields.

Gravity is additive, so the fields smooth each other out where they connect.

Merged gravity fields are smoother.

Water follows the gravity field at some level. In this case, I drew it slightly farther than "light" gravity, but in reality it will be whatever level has enough volume beneath it to hold all the water.

Water with dip because of merged gravity fields having unlevel potential height.

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    $\begingroup$ "for a flat surface" Which of course is a curve. Let's not give the flat earthers any ammo. :) $\endgroup$
    – Glen Yates
    Commented Jul 25, 2023 at 15:31
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    $\begingroup$ @GlenYates the flat earthers aren't using ammo, just stale farts in brass painted wooden casings. $\endgroup$
    – Stian
    Commented Jul 25, 2023 at 16:01
  • $\begingroup$ The second picture shows that gravity works much more like a low pressure system than the classic bowling ball in a trampoline depiction. The greater the gravity - the deeper the low pressure system. So you can think of CNN's "gravity hole" as a local high pressure system and the area completely surrounding it as a bunch of Lows that attract everything towards them. Just like MichaelS's second picture above. $\endgroup$ Commented Jul 26, 2023 at 15:34
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    $\begingroup$ Can you improve the answer by making explicit references to gravity force fields and gravity potential fields? They have different properties and you have used both of them. $\endgroup$
    – Phil Sweet
    Commented Jul 27, 2023 at 0:47
  • $\begingroup$ I find the last image misleading because it suggests the seafloor is not flat. $\endgroup$
    – summerrain
    Commented Mar 9 at 21:21
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(Based on David's answer but maybe a bit more intuitive - this is how I understood it)

The sea level is a surface of constant gravitational potential. Otherwise the water flows from higher to lower potential.

In parts of the world with weaker gravity, the gravitational potential is less at the same height, which means the same gravitational potential exists at a lower height in these places.

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    $\begingroup$ I may be just reading this incorrectly, but can this be reworded to make it clearer? - "is lower at the same height, which means it's lower at the same height" $\endgroup$
    – HandyHowie
    Commented Jul 25, 2023 at 10:27
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    $\begingroup$ @HandyHowie just a brain fart while editing $\endgroup$ Commented Jul 25, 2023 at 13:09
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    $\begingroup$ I think some directionality is reversed here. Gravitational potential decreases from zero (at infinity) as one gets closer to a source, at a rate equal to the local gravitational field strength. If the strength is smaller along some ray from the source, then, the gravitational potential will be higher at a given distance along that ray than along nearby rays, since it has not decreased as much. This now means that one must be at a shorter distance along the peculiar ray to obtain a given value of the potential. $\endgroup$
    – jawheele
    Commented Jul 25, 2023 at 14:52
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    $\begingroup$ Re The sea level is a surface of constant gravitational potential. That's not quite right. It is the geoid (to which the referenced article refers) that is a "surface of constant gravitational potential." Sea level varies from the geoid due to temperature, salinity, and currents. For example, sea level on the Pacific side of Panama is on average about 20 cm higher than it is on the Atlantic side. $\endgroup$ Commented Jul 31, 2023 at 2:25
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I think it's helpful to use thought experiments trying to imagine extreme cases and what would happen.

In this case, imagine the Western hemisphere was solid earth, and the Eastern hemisphere was replaced with a hollow shell with vacuum inside and smooth on the outside, but with the water from the Eastern hemisphere still roughly in place. I would naturally expect all the water from the Eastern hemisphere to flow around the hollow shell towards the Western hemisphere because the gravity of the Western hemisphere would attract it. This would lead to shallower water in what was the Indian ocean.

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