Here is a fascinating video from a recent article by Shireen Gonzaga on the excellent EarthSky website.

The article explains (and the video illustrates) the important concept of the relative gravity differences across earth's surface.

Generally speaking, all objects with mass have gravity. A bowling ball has more mass than a marble, but because the earth itself has so much more mass than either of them, we don't notice the force of gravity which pulls objects towards the center of either of them, because the small force vectors pulling towards their centers of mass are totally overwhelmed by the much larger force vectors pulling towards the earth's center of mass.

However, if we moved the bowling ball and the marble into space, far enough away from the earth or other planets, the force of gravity towards the center of the bowling ball and the marble could each be noticed and measured. In fact, if you placed the marble some distance away from the bowling ball, the force of gravity would pull the marble to the bowling ball due to the bowling ball's greater mass.

A person's mass doesn't change if you move him from the earth to the moon, but his weight on a scale would change. On the moon, which has less mass than the earth, the person would be pulled towards the center of the moon with less force than on earth he is pulled towards the center of the earth. Thus a scale beneath his feet on earth would register the greater pull and on the moon the same scale would register the lower pull -- he would weigh less on the moon, although his mass is the same (his mass "with clothing" might even be greater on the moon, if he is wearing a bulky pressurized space suit).

As the article and video above illustrate, this same phenomenon takes place on earth as well, in areas of slightly greater or lesser mass, although you would need an extremely sensitive scale to detect the differences (your bathroom scale at home won't cut it).

Near a very massive mountain, gravity will pull towards the mountain as well as towards the center of the earth, although the force vector towards the center of the earth will be much stronger than towards the mountain. Above the mountain, the force vector pulling on an object (such as a person) will be larger than above a less-dense area of the earth's crust (you can think of it as the force vector towards the mountain being added to or "stacked upon" the force vector towards earth's center). Thus, a very sensitive scale will register a slightly higher weight for a person above a massive mountain or a dense part of earth's crust, and will register a slightly lower weight for a person above a less-dense portion of earth.

This phenomenon is what the geoid in the video illustrates -- an exaggerated view of the areas of relatively greater and lower density and gravity on the earth, as recently measured by the very precise gradiometer onboard the Gravity Field and Steady-state Oceanic Circulation Explorer (or GOCE) satellite.

In his explanation of the hydroplate theory, Dr. Walt Brown explains that it has been known for decades that there is a mysterious deficiency in gravity beneath the deep ocean trenches of the Pacific. He points to an often-cited work by geologists Robert L. Fisher and Roger Revelle, "The Trenches of the Pacific," in which the authors state:
The most striking phenomenon associated with the trenches is a deficiency in gravity. [. . .] Measurements of gravity near trenches show pronounced departures from the expected values. These gravity anomalies are among the largest found on earth. It is clear that isostatic equilibrium does not exist near the trenches. The trench-producing forces must be acting [. . .] to pull the crust under the trenches downward! cited in Brown 122.
While this might seem to make sense, as a trench is something of the opposite of the mountain we considered above, it does not make sense at all under the theory of plate tectonics. According to tectonics, a trench is created when one massive plate subducts underneath another plate, creating a trench. These plates are literally miles thick (which calls into question whether one could really subduct under another in the first place), so there should actually be more mass where one plate dives beneath another plate, rather than less mass.

However, as Dr. Brown explains in pages 122 to 123 of the 7th edition of his book In the Beginning: Compelling Evidence for Creation and the Flood (available on his website), the finding of a deficiency in gravity associated with the trenches of the Pacific accords perfectly with hydroplate theory's explanation for their origin.

According to his theory, the floor of the Pacific was sucked towards earth's center as a result of the movement of earth's crust away from earth's center under what is today the Atlantic, on the opposite side of the earth, when water and tons of rock were removed by the initial rupture of that created the flood event. This explanation also accounts for the arc-and-cusp shape of many ocean trenches that are very difficult to explain under the tectonic theory (see brief discussion here).

What does all this have to do with the timeline of mankind's ancient past and the evidence of ancient civilizations?

If the features on the earth were not created by tectonics (and there is extensive evidence that calls the tectonic explanation into question), and if they were instead created by catastrophic forces (as Dr. Brown's theory posits and as extensive evidence seems to support), then these geologic features would not necessarily have required hundreds of millions of years to form but could in fact have been shaped within the course of weeks, months, or a few years.

This means that they could have been shaped at almost any time, even as recently as a few thousand or tens of thousands of years ago. (Note as an aside that this possibility radically undermines the Darwinian theory, in that Darwinian evolution requires vast amounts of time as a key ingredient in its formula of random mutation plus natural selection). If so, then the catastrophe could have occurred during human memory.

In fact, I argue that the evidence available to us from ancient myth and archaeology supports the assertion that this catastrophic event did take place during human memory.

Dr. Brown's theory explains that the event he posits would have initiated a major roll in the earth that changed the sky. This would help to explain why those who noticed this major change began to closely monitor the sky and came to understand the phenomenon of precession.

While not generally admitted by conventional theories of man's past, I argue (based on the work of others who have gone before, including the seminal work of Giorgio de Santillana and Hertha von Dechend) that ancient man understood the phenomenon of precession in impressive detail and precision and recorded that knowledge in his mythology and his most enduring physical monuments. The theory of Dr. Brown, which deals primarily with geology and which is therefore not often considered or understood by those studying anthropology or theories of ancient civilizations, could provide (in my analysis) an exciting new explanation for ancient man's grasp of this subtle and sophisticated astronomical concept.

I explain this new possibility in detail in the Mathisen Corollary.