What is "hvergelmir"

In Norse mythology, Hvergelmir ( Old Norse "bubbling boiling spring") is a major spring. Hvergelmir is attested in the Poetic Edda, compiled in the 13th century from earlier traditional sources, and the Prose Edda, written in the 13th century by Snorri Sturluson. In the Poetic Edda, Hvergelmir is mentioned in a single stanza, which details that it is the location where liquid from the antlers of the stag Eikþyrnir flow, and that the spring, "whence all waters rise", is the source of numerous rivers. The Prose Edda repeats this information and adds that the spring is located in Niflheim, that it is one of the three major springs at the primary roots of the cosmic tree Yggdrasil (the other two are Urðarbrunnr and Mímisbrunnr), and that within the spring are a vast amount of snakes and the dragon Níðhöggr.