Coral atolls consist of submerged volcanic mountains, often at great depths below the surface, upon which huge formations of coral have grown.

Scientists and engineers have drilled deep into several atolls and have been able to determine the size of the coral cap and the depth of the top of the volcanic seamount underneath the coral of the atoll. On Funafuti Atoll in Tuvalu, drilling determined that the depth of the volcanic rock was nearly a thousand feet below the surface (see for instance Geology and Hydrogeology of Carbonate Islands, page 582). On Bikini Atoll, the coral cap goes down over 3,200 feet. Drilling has found that the mountain beneath Eniwetok Atoll is over 4,600 feet below the surface in some places.

Reef-building corals cannot survive and build reefs at depths below about 160 feet at the most, because they require sunlight for photosynthesis. Therefore, the great depths of some of the volcanic mountains supporting huge coral atolls poses something of a geological mystery. How could a mountain whose top is over 3,200 feet below the surface support the Bikini Atoll? How could a mountain whose top is over 4,600 feet below the surface support the Eniwetok Atoll? Even the atolls in Tuvalu, whose tops are just under a thousand feet below the surface pose a problem, since corals cannot begin their construction at anything near those depths. A related question concerns the existence of guyots or tablemounts, which are huge undersea volcanoes with flat, truncated tops, possibly sheared off by violent wave action, but often located many thousands of feet below the surface.

As it happens, Charles Darwin was aware of this mystery and proposed an explanation, which (according to this educational webpage from the University of Arizona) is still largely accepted today. He put the problem like this in the Structure and Distribution of Coral Reefs (1842), which can be read online in its entirety here:
What cause, then, has given to atolls and barrier-reefs their characteristic forms? Let us see whether an important deduction will not follow from the consideration of these two circumstances,—first, the reef-building corals flourishing only at limited depths,—and secondly, the vastness of the areas interspersed with coral-reefs and coral-islets, none of which rise to a greater height above the level of the sea, than that attained by matter thrown up by the waves and winds. I do not make this latter statement vaguely; I have carefully sought for descriptions of every island in the intertropical seas; and my task has been in some degree abridged by a map of the Pacific, corrected in 1834 by M.M. D'Urville and Lottin, in which the low islands are distinguished from the high ones (even from those much less than a hundred feet in height) by being written without a capital letter: I have detected a few errors in this map, respecting the heights of some of the islands, which will be noticed in the Appendix, where I treat of coral-formations in geographical order. 90.
Darwin's proposed explanation was simple: the existence of coral atolls is "quite inexplicable, excepting on the theory, that the bases on which the reefs first became attached, slowly and successively sank beneath the level of the sea, whilst the corals continued to grow upwards" (98).

Of course, Darwin was writing before the tectonic theory had even been suggested, and his explanation as to what could cause the mountains that formed the bases for these reefs to sink or subside into the depths below is pretty vague. He basically summed up the proposed cause of the subsidence as "subterranean disturbances beneath them" (98).

Unfortunately, in the ensuing period of over a century and a half, the explanation has not gotten much better. This article from the California Academy of Sciences declares that lower sea levels due to ice ages, followed by sea level rise after the end of an ice age, contributed to the ability of coral to grow to such heights (see the section entitled "Formation of Coral Reefs"). However, as we have discussed in previous posts, no ice age theory can account for 4,600 feet of sea level rise from ice melt alone. Further, if the ice had melted too rapidly and the ocean level had risen faster than the coral could grow upward, Eniwetok and other atolls could never have formed, because the sea-level rise would have "outrun them."

Other explanations are equally vague. The formation of Midway Island, a coral atoll located northwest of the Hawaiian Islands, is often attributed to subsidence due to the weight of the island and the coral, but clearly the Hawaiian Islands would have even more size and weight and should have sunk as well (some theories argue that they will, if we just give them enough millions of years, which function as a kind of magic wand for many theories that are short on details).

This webpage from the US National Oceanographic and Atmospheric Administration (NOAA) shows a helpful graphic of a coral reef growing upwards and a volcanic island sinking downwards, but the explanation of how this takes place is fairly unhelpful: "As the reef expands, the interior island usually begins to subside and the fringing reef turns into a barrier reef." Why does it "usually begin to subside"? This sentence makes it sound as though the reef expanding has something to do with it.

In their description of the significance of Midway Island, the US Fish and Wildlife Service was forced to concede: "Midway is an example of atoll formation, a poorly understood geological process that can contribute to our understanding of the relationship between climate, reef development, and carbon sequestration" (3-4).

However, it is possible that in this case, Darwin was correct about the idea that coral reefs of such great heights grew atop volcanic mountains that once were near or above the surface, and then slowly subsided. While the tectonic theory has trouble explaining why they would subside, the hydroplate theory actually provides a very robust explanation.

Central to the hydroplate theory is the formation of the Mid-Oceanic Ridge, where the basement floor sprang upwards in response to the removal of tons of material by the jetting eruption of the waters that initiated the global flood. This upward movement would not have created a huge air pocket in the middle of the earth, but rather would have pulled or sucked the earth upwards on the opposite side from the bulge -- which is exactly what happened to create the deep basins of the Pacific and Indian Oceans.

This downward motion would explain the characteristic "arc-and-cusp" patterns of many deep ocean trenches in the Pacific, as well as the gravity anomalies beneath these deep trenches, which have far less gravity than the tectonic theory would predict. The violent downward buckling of the Pacific and Indian oceans would have also created massive friction and melting of rock deep under the surface, creating vast reservoirs of magma, much of which either seeped upwards and formed volcanoes and volcanic mountains and islands (note that the "Ring of Fire" marks the edges of the part of the earth that was sucked inwards according to this explanation), or else it would have sunk deeper into the earth to contribute to the liquid outer core beneath the mantle, according to Dr. Brown's hydroplate theory.

As we have explained in previous posts, the sea levels would have been much lower after the events surrounding the flood, but would have slowly risen in the centuries thereafter, as the continents sank downwards. At the same time, the Pacific floor and many of the volcanic formations on it would have also experienced sinking, due to the entire mantle sinking into the liquid of the magma that was created by the intense friction of the event just described.

Dr. Brown explains this process in relation to the formation of seamounts and atolls (which are found almost entirely in the Pacific directly on the other side of the globe from the Mid-Atlantic Ridge, right where his theory says they should be, as well as to a lesser extent in the Indian Ocean):
All the fracturing and shifting deep within the earth produced frictional heating, gravitational settling, and huge amounts of heat and magma. Most of that magma now constitutes the earth’s outer core. [. . .] For years after the flood, much magma escaped upward along faults, especially in the Pacific, which had the fastest-sinking and most fractured portion of the crust. Volcanic cones rapidly rose, many reaching the ocean’s surface, where large waves leveled the volcanic peaks. Over the next few years, the mantle below the Pacific plate sank even further into the growing liquid outer core, because the Pacific plate was loaded with thick, dense magma. That sinking pulled tablemounts down 3,000–6,000 feet below sea level. The tablemount and trench region is several thousand feet lower than the average depth of the Pacific. [. . .] Clustered tablemounts sometimes differ in elevation and depth by 1,000–2,000 feet, so they apparently formed at different times while local ocean depths were changing rapidly. This probably happened during the years after the compression event as the mantle below the Pacific plate sank into the growing liquid outer core. When new cracks permitted magma to escaped upward, seamounts grew from different depths. Therefore, the first tablemounts that formed were usually shorter than tablemounts that formed after the plate had been pulled deeper. Earlier tablemounts were then pulled down farther than those that formed later. Consequently, short tablemounts can be far below sea level, while nearby, taller tablemounts can have their tops at shallower depths. From the online edition; see the full explanation from which the quoted section above was cited here.
Dr. Brown provides much more discussion which supports this explanation of the formation of tablemounts. While Darwin appears to have deduced the correct explanation as well, he could not provide a solid geological explanation for the mechanism that would have caused the subsidence of the areas of the Pacific containing tablemounts and atolls. The hydroplate theory does.

This is yet more evidence that the hydroplate theory should be carefully considered as a possible explanation for the many features we see in the world around us, features that are difficult or impossible to explain with the reigning tectonic theory (numerous others are listed here). The ability of the hydroplate theory to explain such widely varied geological mysteries is truly impressive. As one might expect, if the hydroplate theory is indeed correct, it would also help shed light on numerous mysteries of mankind's ancient past as well, which is the subject of the Mathisen Corollary. The case of the coral atolls should be seen as a very strong argument in its favor.