He's written a series of articles at the REHupa website entitled "The Mysteries of the Hyborian Age." As with most of his work, the articles are exceptionally interesting and enlightening to read, though I have developed serious issues with his interpretations of certain events, particularly those in which he writes of the layout of the Thurian Age (the age of Kull), and his writings regarding the Cataclysm that ended the Thurian and led into the Hyborian Age (the age of Conan).
The biggest issue I have regarding the geography of the Thurian Age is his location of the continent of Mu. He draws from Solomon Kane stories to locate this continent in the South Seas, southeast of Lemuria, which would place it somewhere near where Australia is located. Many more recent Hyborian Age maps use this interpretation. The real problem with this, is that the Solomon Kane stories may not (and probably do not, as they do not fit seamlessly with the Cthulhu Mythos as do Howard's stories) take place in the same universe/continuity with Kull and Conan. In Howard's own stories, he mentions that the islands of Lemuria are formed of the mountain peaks of sunken Mu. This brings to light another mistake made by Rippke, that Lemuria was a continent. In "The Hyborian Age," Howard's seminal essay on his world, he discusses the Thurian Age (though he doesn't call it such):
Known history begins with the waning of the Pre-Cataclysmic civilization, dominated by the kingdoms of Kamelia, Valusia, Verulia, Grondar, Thule and Commoria...The barbarians of that age were the Picts, who lived on islands far out on the western ocean; the Adanteans, who dwelt on a small continent between the Pictish Islands and the main, or Thurian Continent; and the Lemurians, who inhabited a chain of large islands in the eastern hemisphere.
Howard himself clearly mentions that Lemuria was a chain of islands, not a continent. Howard does talk of a "shadowy and nameless continent lying somewhere east of the Lemurian islands." It is my feeling that Rippke is associating this nameless continent with Mu. The problem with this is that Rippke's various articles do not take into account the North and South American continents in any way. Certainly, Howard doesn't name these continents directly; however, his Hyborian and Thurian Ages are supposed to be epochs of the planet Earth, and his descriptions and maps describe only the Eurasian/African/Japan areas of the world. By definition there must be a North/South American land mass to the east of Lemuria and to the west of the Thurian continent. There does not, however, have to be an Australian continent, as that particular continent could've been a part of Antarctica at that time (theoretically). Or, it could've been a piece of the Thurian continent that had yet to break away.
Regardless, the failure to acknowledge the existence of a North/South American landmass forms a major problem in Rippke's theories. Not accounting for this massive continental mass between Lemuria and Thuria (to the east of the former and the west of the latter) is a serious oversight.
It's been well established that Atlantis lay to the west of the Thurian continent, between it and what (in my estimation) are the Americas. Rippke lays out the following geography of the Thurian Age, using Atlantis as the centerpoint:
The Thurian continent lies to its immediate east.
The Isles of the Sunset (Pictish Isles) lies to its immediate west.
Lemuria lies southwest of the Pictish Isles off the eastern Thurian coast.
Mu lies south of the Pictish Isles and southwest of Atlantis.
Kaa-u lies southeast of Atlantis and south of western Thuria.
The Isles of the Sea possibly lie south and slightly west of Atlantis.
Rippke's geography places the Pictish Isles probably somewhere in the vicinity of the Bahamas, but again doesn't account for the Americas. Perhaps the most damning evidence of Rippke's failure to account for the Americas is the following quote, lifted from this site:
I believe that the "trigger" of the Great Cataclysm was an cometary impact in the sea between the small continent of Atlantis and the island chain of Lemuria.
If Lemuria is, as is generally accepted, roughly analagous to where Japan is now (in the Pacific Ocean), and Atlantis is situated in the Atlantic Ocean, how, then, can Atlantis and Lemuria collide without each tearing straight through the Americas? They can't.
A new examination of Rippke's geography must be undertaken. It'll probably take me forever to do it, given all the reading I have to do for school, but I'm going to try and be equal to the task. Hope you all find this interesting. Looking forward to comments.