Hydroplate theory is a creationist hypothesis that Earth once had huge chambers of water sandwiched between the earth's crust and its mantle. It was invented solely to explain the question "Where did the water for the great flood come from?" - in other words, to provide a methodologically naturalistic reason to throw methodological naturalism to the winds. (The obvious answer, that a God capable of creating the universe in a single week can conjure the requisite water out of thin air, was apparently not up for consideration.) It was first proposed in 1980 by Walter Brown (holder of a PhD in mechanical engineering).
According to Brown, before the Flood, the Earth's crust floated on a thick layer of water, above the mantle. Walls and tendrils connected the mantle and crust, allowing the inner and outer reaches of the planet to rotate on its axis at the same speed.
But the fantasy does not stop there; according to this "theory" antediluvian Earth had one super-continent—similar in concept to, but not the same as, Pangaea—that covered about 75 percent of the surface. Oceans, if you could call them that, were really giant lakes (like the Sea of Galilee, the Red Sea, etc.). To maintain consistency they are obliged to argue that Earth's mountains rarely reached more than 5000 feet (1524 meters) above what was then sea level and the highest mountain was probably much lower than 9000 feet (2743 meters).
 How the "Great Flood" occurred
When it came time to flood Earth, God cracked the crust, releasing the water (like a soft-boiled egg). These cracks formed underwater mountain ranges like the Mid-Atlantic Ridge. The subterranean water sprayed upward with such great force that it caused worldwide rainfall for 40 days and 40 nights (and made the comets, and anything else they don't understand). The water left its subterranean encasements so quickly that many land masses immediately began to sink. These rapidly sinking land masses uplifted other land masses, thus causing deep-sea fossils to appear at the tops of mountain peaks. (Creationists always point to "mountaintop fossils" as proof of the flood — naturally wanting to wish away the vast amount of evidence for slow-motion tectonic forces taking millions of years to raise mountains from the ocean floor.)
 Results of the Flood
The proponents of this pseudoscience claim that many of the sunken land masses remain sunken today, forming the world's ocean floors. All of these sudden movements caused the super-continent to develop tectonic ridges and mountain ranges that run parallel to Earth's coastlines. These movements also created, in the eyes of creationists, an illusion that led scientists to develop the theory of Pangaea.
The ocean floors took less than six months to sink to their current depths. The flood waters then filled these troughs, creating the oceans and drying out the deluged land.
This theory is also being prepared as a pre-emptive strike against any living microbes that might be found on Mars or other parts of our solar system. Creationists are claiming that some mud might have gone flying out into space, and landed on another planet.
 Biblical origins
In the six hundredth year of Noah's life, in the second month, the seventeenth day of the month, the same day were all the fountains of the great deep broken up, and the windows of heaven were opened.
And the rain was upon the earth forty days and forty nights.
 Under-Mantle Ocean "Vindicates" Hydroplate theory!
(Spoiler: No it doesn't.) Recently, a discovery was made from a battered ringwoodite that survived a trip from the mantle, indicating a long held theory that there a very, very large amount of water that's trapped in a really distinct layer in the deep Earth. Creationists, ever the straw-graspers, jumped on this discovery and started spamming this discovery everywhere as "proof" of the validity of hydroplate theory.
However, it should be pointed out that those who post and point to this discovery as "proof" are committing a common error among creationists: Not actually reading the article beyond the title. If they had, they would find that first of all the notion that water could simply "explode" out from the mantle is quite frankly ridiculous. Secondly, unlike the title alone suggests, scientists did not find some "ocean of liquid water just sitting in this transition layer waiting to be spilled out on a moment's notice", but rather incredibly high pressure concentrations of non-liquid ringwoodite that contain "water" in the form of hydroxide ions.
What they found was not really an "ocean of water below the crust" but rather a ring of ringwoodite between the upper and lower mantles. Hardly anything that could have caused a global flood through plate tectonics or any other volcanic mechanisms.
The only thing this discovery really "proves" for creationists is just how desperate they are to vindicate themselves. Of course, at this point it should be pretty obvious.
 See also
Links to Creationist web pages that discuss this theory and other similar theories:
- Glenn R. Morton's rebuttal to Walter Brown's Hydroplate Theory
- A YouTube video about Hydroplate Theory
- ↑ No, seriously. This is the entire basis of the theory.
- ↑ http://www.newscientist.com/article/dn25723-massive-ocean-discovered-towards-earths-core.html#.VNjTj_nF_uN