The Grand Canyon is often included on lists of the "Seven Wonders of the Natural World" (following on the tradition of creating lists of the "Seven Wonders of the World," a tradition which started in antiquity).  It is truly one of the most massive canyons on earth, stretching well over 200 miles, over the course of which its widths span from four to an incredible eighteen miles across, and reaching an average depth of a mile from the rim to the riverbed far below.

The amount of earth that had to be removed to form such an enormous abyss is truly staggering.  The US National Park Service web page lists the volume of the Grand Canyon as 5.45 trillion cubic yards.  This is an almost-inconceivable volume of dirt that had to be removed.  

Where did it all go?

Walt Brown, the originator of the hydroplate theory, who devotes an entire chapter of his book (available online and in print) to the Grand Canyon, recognizes the significance of this question.  He notes that the volume of sediments that had to be displaced totals about 800 cubic miles!  

He also explains that most conventional theories for the formation of the Grand Canyon, such as the idea that the Colorado River slowly eroded this massive canyon (averaging ten miles wide and one mile deep for well over 200 miles) have a real problem explaining where all that dirt went.

The Colorado River empties into the Gulf of California (the body of water between the Baja peninsula), and Mexico itself.  If that dirt was gradually eroded, there should be a massive delta where the river meets the gulf, but the delta there is tiny, containing not even 1% of the volume of dirt that must at one time have been removed from the Grand Canyon (see for instance point 20 on this web page from Dr. Brown's book).

Dr. Brown relates the story of one of the Grand Canyon's most colorful characters from the end of the nineteenth and early twentieth centuries, John Hance (known as "Captain" John Hance, or sometimes "Cap").  He became famous for regaling visitors with his tall tales, including his explanation of how the mighty canyon came into being.  Quoting a description of Captain Hance's famous account of the canyon's origin, given by former Arizona governor and former US Secretary of the Interior Bruce Babbitt, Dr. Brown relates:
Children loved John Hance, and to them he always explained how the canyon came into being.  "I dug it," he would say simply.  This story worked well for years until one little four-year old girl asked seriously, "And where did you put all the dirt?"  Hance had no ready answer; he never used that story again.  But it bothered him the rest of his life, and when he was dying he whispered to his waiting friends, "Where do you suppose I could have put that dirt?" (from this page in Dr. Brown's online book, quoting Bruce Babbitt -- see footnote 4. on this page).
For more of John Hance's deadpan tall-tales, see this description of the colorful Grand Canyon guide.  Apparently, the question of where all that dirt went made a deep impression on Captain Hance, and troubled him to the end of his days.  It is a question that conventional geologists have yet to answer.

However, the hydroplate theory of Dr. Brown provides an answer for the question of where all those cubic miles of dirt ended up.  As related in previous posts, and discussed in greater detail in his book, Dr. Brown's hydroplate theory argues that the Grand Canyon was not carved by the mechanism of slow erosion by the Colorado River over millions of years, but rather that it was created in a relatively short period of weeks or months by the catastrophic breaching of two enormous inland seas, each one of which were left over from a world-wide flood.  Previous posts pointing to evidence that makes the conventional theory difficult to accept but which support Walt Brown's theory for the formation of the Grand Canyon include:
If the Grand Canyon is a product of a massive, high-volume and high-intensity outpouring of millions of tons of water from two huge inland seas left over from the world-wide flood (you can see where Dr. Brown believes these two huge water bodies once stood on this map in his online book), then the final resting place for all that dirt would be very different than if the dirt were removed gradually over millions of years by a relatively small river.

In fact, the sudden breaching of two enormous water bodies of the size described by the hyrdoplate theory would have removed even more dirt than was in the Grand Canyon, as massive as that is.  According to the hydroplate theory, the breaching of these two inland seas removed at least 2,000 cubic miles of sediments above what is now the Grand Canyon, in addition to the 800 cubic miles of sediment that had to come out of the canyon itself.  You can read in his book how the removal of all that sediment caused the layers below to arch upwards, a phenomenon whose evidence is clearly visible in the geology of the region of the Grand Canyon, and in places to crack (Marble Canyon was caused by this upward arching and subsequent cracking motion).

All those cubic miles of sediments were washed away by the violent release of the two huge lakes, and they swept along until they dumped into the sea -- in this case, they dumped into the Gulf of California, where the Colorado River still meets the sea today.  Along the way, many sediments were deposited into the region between the Grand Canyon and the Gulf of California, but a huge quantity of them dumped into the gulf and they are still there today.

The image below, from Dr. Brown's book here (see section 13, "Missing Dirt"), contains modern three-dimensional imagery of the Gulf of California showing where all those sediments ended up.  Dr. Brown's caption for the image reads in part as follows:
Here's the Dirt.  It's right where we would expect it, if we understood the Grand Canyon's rapid and violent formation.  Hidden beneath the flat floor of the Gulf of California are at least 6,000 cubic miles of sediments.  That basin, bounded on the south by the largest islands in the Gulf, has an area of 15,000 square miles (220 miles long and 60-100 miles wide).  Sediment depths are up to 1.2 miles thick!  About half the basin's sediments were rapidly transported from the Grand Canyon (on the figure's northern horizon), along the path now occupied by the Colorado River.
Why is the Northern Basin's 15,000-square-mile floor so flat?  Within weeks, a few thousand cubic miles of sediments were swept into the basin.  The larger particles settled out first, near today's shoreline.  Finer particles settled out last, but until they did, the muddy water, because it was denser, flowed to the basin's deeper regions where the mud eventually settled, flattening the seafloor.
You can see water depths for the various parts of the Gulf of California in this 1956 study of the feature, "Oceanographic and Meteorological Aspects of the Gulf of California," by Gunnar I. Roden.  The excellent bathymetry charts on pages 22 and 23 of that study clearly show that while the northern portion of the gulf (where Dr. Brown's theory says the sediments were dumped) has depths below 200 meters, the rest of the gulf reaches depths of over 2,800 meters!  In other words, if Dr. Brown is correct -- and the evidence from the Gulf of California seems to support his argument -- then the depths of those sediments are truly astonishing.

This evidence is just one more of many pieces of evidence surrounding the formation of the Grand Canyon which appears to refute the conventional explanations and support the explanation put forward in Dr. Brown's hydroplate theory.  The evidence for the hydroplane theory from the Grand Canyon alone is extremely compelling, but that is just one geological feature among literally several hundred more that Dr. Brown examines in his book, all of which contain evidence which appears to support his theory.

Based on all this evidence, the conventional theories seem about as plausible as Captain John Hance's wry explanation for the origin of the Grand Canyon.  Or, to say it another way, Captain Hance's explanation appears just as good as the stories that park rangers tell visitors to this day regarding the origin of this "natural wonder of the world."  (But where did he put all that dirt?)