Where Did That Entrance Come From?

I have a dungeon on the brain.  I'd like to crowdsource this part:

The entrance to a long long lost dungeon was recently found. What hid/obscured the entrance in the first place? I'll begin, and you can add to the list from there:

-Powerful illusion that just expired...

Your turn!

(By the way, comments should be wide open now. Please let me know if they aren't.)

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