It rains...
Merely a leak.
The lake has tides without a moon and the Lowerdark has winds without atmospheric pressure. The Underdark is a big, magical place with its own set of laws. I'm sure this is as intended.
It is as intended as leaking, but is a minor feature, so if it really bothers some of you to have leaking on you, I don't mind taking it off.
I thought it was genuinely raining. Sorry LPFF.
Also, I love the rehash of the old troglodyte quest. :)