Well of Ancients
The Minotaur sage Rahm mentioned this well when he was rescued from the Beggar King’s dungeon. He said the Beggar King was looking for the well to communicate with the Nine Hells.
The PCs found a well that had two levels of protective arcane sigils preventing entry (or escape?), but both protective walls have now been broken, along with their arcane sigils. The well itself is inscribed by a pentacle. Exploration of the well revealed that after about 30 feet the walls appear to dissapate into a mist. It is presumed that this is the Well of Ancients.
Further exploration was deemed unwise at the time, although Ian really wants to just jump in and see what happens.