UV-stabilised low-density polyethylene (LDPE) sheets with fully welded seams make a durable, low-maintenance pond liner for water features, ornamental ponds and the like. Rigid plastic is commonly used if the excavation is square, the surfaces are even and the ground is stable – for example, if the pond is contained within a concrete-lined pit. Semi-rigid plastic can also be used for irregular shapes, uneven contours or unstable soil.