This is a preformed 115 gallon pond. I clean it weekly with a sump pump and refill with a garden hose. Ponds dug right into the ground soon become smelly, muddy messes and mosquitoes abound!
A pond liner might be better for inground pond as long as you clean it often! I find the preformed to be the easiest of all and you can drop a horse trough heater in it for winter months.
I buried the deepest part and let the shallow edges rest on the ground. Then placed stones inside on shallow ledges so call ducks could climb in and out.