The feature on the right side of #3 is very much natural. There are several such "scoops" on the property, a few much larger.
What may have caused this, if anything? My guess is Bison. There were Sioux tribes in the area who camped along the Dismal. A tale is told that Buffalo Bill engaged in an "Indian battle" on the property. Arrowheads can be found - quite common out this way. The Souix tribe was nomadic and moved all over the area, camping near water, covering several states. We are 2ish hours from Fort Robinson, where the final "Renegade Sioux" surrendered, where Crazy Horse incarcerated and murdered. The Wounded Knee Massacre "battlefield" is even closer.
The property has been used long before "we" arrived. Just off of Tom's 16th green is an anchient "buffalo run" which crosses the Dismal providing access through the "steeps". I'm guessing it's been there for centuries, or more.
There is a very interesting and massive feature to the left of 17 Nicklaus. Very large and very much the "big brother" of the scoop left of #3. Once of the coolest green sites I have seen anywhere. Today, home for deer and other critter's. Most people never even notice it - but, if you have look, it is mesmerizing.
As far as #3, I'm certain you don't want to hit in there and love what Tom and the crew have done. The course has really come alive and I believe it will be one of Tom's most special. The holes (and bunkers) really fit the land and its spirit. If the wind would lay down, Don could begin to seed her in. Jaeger Kovich worked yesterday in ski goggles - odd in that it was quite warm.
As far as legend, I rather like the meteor strike. Or a Ghost dance hall.