Wed Nazir 53

Tosfos brings the rules of how Tumah manifests itself in the grave. If the grave is not hollow a Tefech high, then Tumah only rises on top of the actual body. If it's hollow a Tefach, Then if it's completely closed, then Tumah rises through out the length and width of the grave, even in the portion that there is none of the body under that area. However, if the grave is not completely closed and there is an opening where the Tumah can escape, so there is Tumah above the grave.