Q.Sunken Garden
My garden and surrounding gardens are in a very flat area. My partner and I are looking at options for creating something different and I am in favor sinking part of the lawn by about 12 inches to create a garden with different levels. The land does not become waterlogged when it’s wet, but my partner is concerned that it will become waterlogged when sunken, or even a small swimming pool if I do.
Do you have any suggestions.
Thanks,
Gavin

Here is an excellent article to help you get started with your landscape project.
https://www.gardeningknowhow.com/special/spaces/sunken-gardens.htm