how many people can know before it is no longer considered hidden?
the hidden source of the impurity of the deep. someone will eventually see where the bodies are buried.
(impurity of the deep is a strange category of impurity. it is when someone has become impure due to hidden source and it is only revealed once the impurity has spread by contagion. this page discuss how many people need to know where the body is before it is no longer a hidden secret. and if a corpse was found lying widthways under a road.)