Der, die or das?
neuter noun · secret · level A2
die Geheimnisse - in the plural, every German noun takes the article die.
No guessing needed: -nis → das · 0.5% (but Kenntnis, Erkenntnis are die) "Geheimnis" follows the rule Nouns ending in -nis, which covers 11 words in the Genau course.