It looks to me like indeed it's a "cove" for plants and/or statuary of some sort. I see some PERLITE in those rocks-- typical planting soil amendment.

You could put some sort of spitting statuary in there surrounded by some plants. I'm not sure what zone your in though. You could even leave some plants potted, so you could remove or rearrange when needed.

good luck !

carlos
--my ponds--