There are two lines (a normal one and one for Khajiit) for the topic "little secret" filtered to Andothren that talk about the old Issurnawia Mine interior, which is an abandoned diamond mine that can only be navigated by levitation due to collapses

The new interior however is just a fairly standard smuggler cave (https://www.tamriel-rebuilt.org/claims/rr-cleanup-rro-issurnawia-mine-ne...), which obviously doesn't quite fit the way it's described in the aforementioned dialogue