Norse cosmology gives Yggdrasil three roots, and a spring beneath each. One reaches into Asgard, the world of the Aesir, and that is Urdarbrunnr, the Well of Urd. Another reaches into Jotunheim, the world of the giants, and beneath it lies Mimisbrunnr, Mimir's well, whose water grants wisdom and whose keeper is the being Mimir, from whom Odin buys a drink at the cost of one of his eyes. The third root reaches into Niflheim, over the spring called Hvergelmir, from which most of the world's rivers are said to rise.
Modern retellings frequently blur the second and third of these, and sometimes fold all three into a single well, but the Eddic sources are consistent in keeping the Well of Urd and Mimir's well distinct: different roots, different waters, different governing figures, the Norns at one and Mimir at the other. The confusion is understandable, since both are described in similar terms and both matter to the tree's survival, but the atlas keeps the sources' own distinction rather than the popular composite, so that the well the Norns tend and the well Odin drinks from remain two different places doing two different kinds of work.