I built this for my mother when I should have gone to my friend’s second wedding in NYC. Dug the hole, used rebar and concrete for the frame, then Thoroseal to prevent leakage. The large rock under the waterfall is below the waterline. The waterline should be on the material used for the framework (concrete + Thoroseal). It seemed like a really cool idea when I built it, but started leaking over the years. I sealed with concrete seal, but would not recommend doing this.
NOTE – Use an in-line UV light to kill the algae! My mother insists on using bleach – the reason for the water’s color in the picture.
BONUS – Here’s my mom digging the hole.