Something that I saw once was a pool that was on top of level area that the people built up not sunk down. They used retaining wall blocks and back filled the area. I did the opposite, I dug out a level surface for my pool. I saw the other way after I had the pool installed or I may have considered it. I thought that the proper thing to do was to dig into compacted dirt. It had an unique look to it and I kind of liked it.

I guess there are pools that can be sunk or partially sunk, I would worry about rot though unless specifically made for that sort of environment.