Chinese police are urgently searching for a quantity of radioactive material stolen from a construction site.
The football-sized lump of Caesium-137 may have been mistaken for scrap metal, the official China Daily said.
The material explodes when brought into contact with water, and can cause blood diseases, tumours and birth defects, officials warned.
Officials in central Shaanxi province have asked anyone who finds it not to open it and to put it in a safe place.
Some 120 police officers and 80 local officials have started searching an area of more than 2 square km around the site of the theft, and have offered up to 5,000 yuan ($602) for clues as to its location, according to the China Daily.
Caesium-137 has a wide variety of uses, from food and medical sterilisation to survey gauges and batteries.
It is not clear what it was doing on a construction site in Pucheng County, Shaanxi.
Zhu Guoying, at the Shanghai Radioactive Materials Research Institute, told the Associated Press that the amount of danger from the Cesium would depend on its purity.
"In a worst-case scenario, it could severely pollute the environment in the area if were melted as waste metal," she said.