User:Average/Ruins of Undermountain
Vast, vast tunnels and endless (abandoned?) rooms still filled with various arcana, wizards and spooks of long ago used the energy of the mountain ranges above to brew potions of alchemy and spells of wizardry during the First Age. You take it for granted now, but once upon a time, before stability of the realms was made the magic of the realms was an endless battle for power, a battle for magic itself.
What remains here are mostly old remnants of those battles, along with shrines to the dark powers that once guided these men. Traps abound as wizards would hide their secrets from each other. If you can manage the cobwebs and pitfalls, you might make it back out with what you found. Otherwise, there may still be old wizards and spells guarding the place. I think no one really knows. Perhaps you will find out.
There are rumors of a secret tunnel from the Mine in Phandelin, otherwise you'll have to go looking down in the filthy gutters in the streets of Waterdeep. If you find yourself in the Abyss, you may find some passageways there as well. Try Gracklstuoh.