unexplained

A death whose cause is not immediately known.

A situation where someone has vanished without a known reason.

необъяснимая болезнь

An illness for which doctors cannot find a cause.

Болезнь, причину которой врачи не могут найти.

An absence for which no reason is known.

необъяснимая тайна

A mystery that has not been solved or understood.

Тайна, которая не была раскрыта или понята.