What is a pond liner?
Answer:
Anything that holds water indefinitely can be a pond liner. 40 mil butyl rubber is the best. You can cut it to order, it lasts well over 20 years and is my choice for ponds.
Concrete not such a good choice because it's brittle and has a problem with temperature extremes.
Flexible PVC is cheap and widely used, but does not hold up at all.
Preformed liners are popular, but just like PVC is only good for a short time.
To find more pond information, go to pondlady.com