Nimueh's Cave

The sorceress Nimueh frequently performs her most powerful magic within the confines of a cave. She hatched an Afanc and delivered it into Camelot's water supply from this cavern, as well as monitoring the aftermath of her various machinations from a stone scrying fount in the cave. (The Mark of Nimueh; The Poisoned Chalice)

The location of the cave is currently unknown. Vast caves exist beneath the forests of Balor, and Nimueh is clearly acquianted with these caverns, as she lured Arthur Pendragon into their depths. Another notable cave system exists beneath the Castle and city of Camelot itself; the Great Dragon is imprisoned in a massive chamber, and the burial vault resides deep beneath the citadel. (The Dragon's Call)

Nimueh also has access to these burial vaults. (Excalibur)