Bifrost

"The appearance of a rainbow is a definite statement from a deity.  Its appearance means either that the deity wants to converse with a mortal, or that the deity wants the mortal to undertake a quest.  In Norse legend, men and women of power could travel to Asgard merely by touching a rainbow (part of the bridge Bifrost) and wishing to be there."

-- Deities & Demigods, p. 10.