alternative to plastic pond liner?

[terracore]

Four years ago I used a cheap blue poly tarp (it wasn't even new) thinking it would hold up for a few weeks so our ducks would have a place to bathe and swim around in circles. Four years later its still there holding in the water, having held up to the ducks, geese, and even a big dog. Somehow it attracted bull frogs, dragon flies (including larvae), even african snails go there to drown. Every day I see water is still in there I'm surprised. There isn't even anything cushioning the plastic from the sharp lava rocks. Usually people put old carpet or something under a liner.