That sounds like an excellent spot for a mulberry tree. Right in the middle. Shade and food all summer. Full sun in the winter.Dexters pen is 40 ft wide and 44ft long. 3 sides have a 6 ft wood fence and the side that faces the inner yard has a 4 ft picket fence with dbl railway ties at the base. He is only 4 1/2 pounds and 10 inches long so he has room to grow.
Just trying to find good hides for him. No shade trees in his enclosure.
You can plant all sorts of good stuff in there if you encircle it with cinderblocks or something similar. Once the plant/bush/tree/cactus gets established and has a thick enough trunk on it, you can remove the blocks.
By all means, try the underground retreat. If you make sure it is pointed down hill, you can reduce the flooding danger.