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| ⇒ тайна, покрытая мраком | |||
| sealed book (natasha396); closed book (a subject or problem that someone does not know about or understand. Например: Mathematics has always been a closed book to me. teterevaann) | |||
| one of the secrets of the seven seals (george serebryakov) | |||
| closely guarded secret (Alexander Demidov); bunch of unknowns (из корпоративного видео, подразумевалось нечто неизвестное Ася Кудрявцева) | |||
| тайна за семью печатями: 2 phrases in 1 subject |
| Idiomatic | 2 |