In addition to high-level waste problems, there are numerous examples of existing disposal sites containing low level waste which have been leaking radiation into the environment. Drigg in the UK and CSM in LeHague, France being just two. No guarantees can be given that waste will remain isolated from the environment over the tens to hundreds of thousands of years. There is no 100 % reliable method to warn future generations about the existence of nuclear waste dumps. An example of where industry plans, to safely store nuclear waste, have been exposed as flawed is the proposed dump site at Yucca Mountain in Nevada, US. After nearly 20 years of research and billions of dollars of investment, not one gram of spent fuel has been shipped to the site from nuclear reactors across the US. Major uncertainties in the geological suitability for waste disposal at the site remain. In the meantime, most nuclear power plants in the United States have resorted to the indefinite on-site dry cask storage of waste in steel and concrete casks.