Abstract: A cavity in the earth such as a reservoir, tunnel or the like is lined with a flexible, fluid impervious sheet such as a rubberized fabric supported by a grid-like skeleton structure having distorted elongated resilient structural members which are under tension and press the liner against the walls of the cavity. The members of the skeleton structure are made of a material which resists permanent distortion. The liner is disposed between the skeleton structures and the wall of the cavity and is fastened at spaced points to the wall of the cavity.