Irradiated earth.

Ship wrecks are a very expensive proposition to raise and bring it ashore. Many ship wrecks lie untouched in the bottom of the ocean. Many wrecks of metal hulled warships, built before the Second World War are popular site for divers. They dive to salvage small pieces of steel, which are then used in the manufacture of radiation sensing scientific devices like Geiger counter. This steel is a precious resource because it was produced before the Second World War, and is the world's only source of steel which is free from the possibility of nuclear contamination. In other words, thanks to all the testing and dumping of nuclear waste, there is a very good chance that any metal that you use will have some quantity of radiation in it.