We just built a brand new home in a brand new subdivision. Our soil is black clay. We have no trees within 100' (if not more) that the location. The home site was graded and we have yet to find anything other than clay within the soil (no rocks, sticks, brick, etc). I've built 3 other tiny garden ponds in the past and never used a underliner. Now that I'm building in a larger scale (estimated at 3,000 gallons), I want to make sure I do everything right. Is an underliner an absolute necessity? Why is one used? Thanks!