The roots of the world ash Yggdrasil rest in three wells -- Hvelgimir, the source of all rivers; Mimir, the well of wisdom; and Urd, seat of the Norns and council place of the gods. The dragon Nidhogg lives in Niflheim near Hvelgimir.  When he tires of eating the bodies of the dead, he gnaws the root of Yggdrasil, hoping to kill the mighty tree. Ratatosk, a mischievous squirrel, travels up and down Yggdrasil carrying insults between the Great Eagle that roosts in Yggdrasil's crown to Nidhogg, at its lowest root. It is unclear if Ratatosk is actually carrying messages or if he is inventing the insults himself.