[A mosque with bathrooms beneath it]

Answer (and Allah grants success): It is established in legal maxims that “the layman’s school is the school of the one he follows.” On this basis, denunciation is not required. In the marginalia to al-Azhār it is mentioned that the author of al-Wāfī said: “It is valid for there to be something endowed to Allah beneath the mosque.”
Al-Kāfī states: “If the benefit returns to the mosque—such as wash areas and the like. And it is said that Bayt al-Maqdis has wash areas beneath it”. End.
As for entry by one in janābah and relieving oneself: if the founder placed the mosque upon the site of a previous mosque, expanded it, and then built toilets beneath it—that is not permissible. But if the founder established the site from the start, first building the bathhouses, and then constructing the mosque above them—it is permissible to enter those facilities for one in janābah and for relieving oneself; for the lower level was made only for that and was not intended as a mosque. And “the layman’s school is the school of the one he follows,” so there is no objection.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1