Is it permissible to pray in a mosque built over a cemetery, most of whose graves have been removed and its ground leveled, knowing that some graves still remain under the mosque and that the mosque is elevated above the ground of the cemetery?

If the graves beneath the mosque are not visible, then the prayer is valid and not invalidated by their presence, because the prohibition of praying in a place containing a grave pertains to visible graves, not hidden ones. However, if the mosque was built after the grave, then either the mosque must be removed or the trace of the grave must be removed, for a mosque built over a grave is not to be prayed in, neither for obligatory nor for supererogatory prayers.