das Mysterium
mystery
Something that is mysterious, inexplicable, or difficult to understand; a riddle.
Das Verschwinden des Flugzeugs bleibt bis heute ein ungelöstes Mysterium.
The disappearance of the plane remains an unsolved mystery to this day.

mystery
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mystery
Something that is mysterious, inexplicable, or difficult to understand; a riddle.
Das Verschwinden des Flugzeugs bleibt bis heute ein ungelöstes Mysterium.
The disappearance of the plane remains an unsolved mystery to this day.

The noun 'das Mysterium' (mystery) is closely related to the adjective 'mysteriös' (mysterious). If something is a 'Mysterium' (mystery), then it is 'mysteriös' (mysterious).
Sein plötzliches Verschwinden war sehr mysteriös.
His sudden disappearance was very mysterious.
Although 'Mysterium' means 'mystery', it is not used in German for the crime-story genre. A crime story is called 'der Krimi' in German.
Das Bermuda-Dreieck ist ein bekanntes Mysterium.
The Bermuda Triangle is a well-known mystery.
In general usage, 'Mysterium' (mystery) is often combined with verbs such as 'bleiben' (to remain), 'sein' (to be), or 'lösen' (to solve). Typical adjectives are 'groß' (great/big), 'ungelöst' (unsolved), or 'ewig' (eternal).
Wie die Pyramiden gebaut wurden, ist bis heute ein großes Mysterium.
How the pyramids were built is still a great mystery today.
You can recognize the religious meaning from words such as 'Glaube' (faith), 'Gott' (God), or 'Kirche' (church). In this context, it describes a truth that surpasses human understanding.
Die Eucharistie gilt in der katholischen Kirche als Mysterium des Glaubens.
In the Catholic Church, the Eucharist is considered a mystery of faith.
miracle / wonder
'das Wunder' and 'das Mysterium' can both have to do with something inexplicable or mysterious. A 'Wunder', however, is usually an event that seems extraordinary and is often understood as divine or supernatural. A 'Mysterium' is more of a mysterious thing or a puzzle that one does not understand. It does not have to be something positive, and it does not have to be an event. The words can be confused because both say: this is hard to explain. 'Wunder' fits better for an astonishing event, while 'Mysterium' fits better for an open question or a secret.

Warum das alte Radio plötzlich wieder funktioniert, bleibt ein Mysterium.
Why the old radio suddenly works again remains a mystery.
Viele Menschen sahen in der unerwarteten Rettung ein Wunder.
Many people saw the unexpected rescue as a miracle.