It creates a site named "The Deepest Cave of the Province" there, which provides 1 earth gem per turn. It also casues a troglodyte attack, varying in number from just a few to almost 20.
I once got 10 mages, that was supposed to reinforce my main force, killed by this event. 2800 gold for one lousy earth gem per turn. Needless to say I didn't consider it a "good" event that time.