Grotto of Ephesus
(The). The test of chastity. E. Bulwer-Lytton, in his Tales of Miletus (iii.), tells us that near the statue of Diana is a grotto, and if, when a woman enters it, she is not chaste, discordant sounds are heard and the woman is never seen more; if, however, musical sounds are heard, the woman is a pure virgin and comes forth from the grotto unharmed.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894