Latin Help

My vaunted education fails me. I need to translate the phrase “Great Secret of the Worlds” (note the plural) into Latin. The sense is that the secret is that there is more than one world. It is not referring to a secret kept by the worlds, in case this different sense would be declined differently.

Would it be magnum mysterium mundorum ? Or do I have a pesky declension wrong?

I ask my wise and well educated readers for help.

Please read and support my work on Patreon!