The "rock" in which the WIPP repository is located is salt (the Salado formation). Salt is fairly plastic, and as rooms are excavated, the pressure on the ceilings pushes them (the ceilings) down. they can be stabilized temporarily with ceiling bolts, but after s few years they collapse and burty the waste. Ruth -- Ruth F. Weiner ruthweiner@aol.com 505-856-5011 (o)505-284-8406