Is it possible this is the remains of a dinosaur. Looks like the cavity of a large animal. Real just kidding. I would b tempted to try and keep tht feature visible if possible. You could ad a light to the inside. How wide is it. Can you take some pictures alil farther back from the opening. It's hard to tell the size of this.

The people who say I can't do it can just sit the @^#% down and watch me. Friends call me Rusto I also subscribe to pond boss mag. http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=504716#Post504716