Known also as the Well of Fate (Urdarbrunnr), and guarded by the Norns; this is one of three wells providing water to Yggdrasil, or the World Tree. The Well of Wisdom (Mímisbrunnr), guarded by Mimir; and the Hvergelmir (Roaring Kettle), the source of many rivers, are the other two. 

All three wells lead to the realms of Asgard, Jotunheim, and Niflheim, which are located deep in the underworld.