Prayer: Digging or Demolishing to Purify a Mosque
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 888
If a place in a mosque becomes impure and it cannot be made pure without digging it up or demolishing it, the impure place must be dug up or demolished provided that it is only a little area, or if rectifying the disrespect caused to the mosque is dependent on digging up or demolishing a large area; otherwise, demolishing it is problematic [i.e. based on obligatory precaution, it must not be demolished]. Furthermore, it is not obligatory to fill the place that has been dug up or rebuild the place that has been demolished. However, if something like a brick of the mosque becomes impure, then after it has been washed, it must be put back in its original position, if possible.