The Nemesis Well is in the hidden city of Osibu, located deep within the Screaming Jungle of the Mwangi Expanse. It is guarded by the ancient arboreal regent Dimari-Diji. The well itself is older than the city that stands around it.
It is thirty feet across, and its depth is unknown. Some people experience a faint chill radiating from the well. Once every five years, the well begins to whisper. The following night the people of Osibu lock their doors, flood their homes with light, and try not to sleep. Those who do sleep have bad dreams, and wake with the feeling that some ancient, hungry creature is trapped in the well. Anything that enters the well is never seen again.
One legend is that the well goes right through the world, opening out again on the other side of the planet. Another is that it provides a one-way route to the prison of Rovagug. On the strength of the latter theory, a priest of Rovagug threw himself into the Well in 4470 AR.
For additional as-yet unincorporated sources about this subject, see the Meta page.