Walipini or underground greenhouse - actual experience

I am looking for actual experience with walipini or underground greenhouse.
What worked? What didn't?
Failure or success in general?
Tips / tricks. Really want to do this, but only if it makes sense.

https://greenhouseinthesnow.com/

I have a question that seemed appropriate for this thread.  

Does anyone have experience or links to stories of building a walipini greenhouse in cold climates in flood plain or high water table locations?  Maybe even a permies member in the Red River Valley of the north-central US and southern Canada (Manitoba)?  The frost depth is around 5 feet deep and the water table pretty high.....and it's flat as a pancake.  I've certainly seen examples of walipinis in our region, but none in such a flat, wet area.  Perhaps one just needs to build on ground level and berm up soil around the non-south-facing sides of the structure?  This would allow the interior to stay a bit drier but at the expense of the insulation/geothermal benefits of being lower into the ground. Experiences or ideas welcomed....and if there is a thread already on this topic in the forums, please let me know.  Thanks!

I researched this quite heavily and common consensus is that walipini work really well in areas near the equator, but not well at all in places further north, like here in Wisconsin.  The sun is very low in the sky here in the winter months and the sunken nature of walipini won't allow the sun to strike the growing bed.  I think a lot of people writing about walipini are simply guessing without having talked to anyone that has tried it.  Here is an excerpt from a webpage that I won't name.  

"When you dig down even just four feet, the temperature changes dramatically. Frost lines generally are three to four deep, so a six to eight foot walipini is completely protected from frost. One walipini owner claims that his greenhouse keeps pretty steady 70 degree Fahrenheit or warmer temperatures when it’s 10 degrees outside. Because underground greenhouses are warmed by thermal mass on all sides, you really can’t lose any warmth compared to a traditional greenhouse."

This is decidedly silly.  A six to eight foot deep spot in the ground that is covered by six to eight feet of soil is completely protected by frost.  Once you remove all the soil, the ground in the hole is now protected by a single piece of plastic, and is in no way completely protected from frost.  Additionally, I would challenge anyone to build a greenhouse that doesn't have an actual heat source in the building that can maintain 70 degrees "or warmer" in 10 degree weather.  It just isn't going to happen.  It can't be done with heavily insulated greenhouses with heat batteries, how could it be done without any additional heat source?

The DWG (Deep Winter Greenhouse) developed by the University of Minnesota takes advantage of heavily insulated walls and a heat battery for use in these climates.  Cost is the major issue.  The DWG saves a lot of money on heating, but is still expensive to build.

I'm interested in the passive greenhouse being trialed by Paul and company but haven't purchased plans for it at this point.

I found this useful doc online. I like that it specifies where the walipini should be lined up according to Winter Solstice. I'm in the NC Appalachian mountains, and though we technically are riding a temperate rainforest, it drops into the 20s a lot at night in the winter. I am really considering one of these!