From Ardrana

One magnificent feature of the planes is the great tree Yggdrasil, the World Ash. Rising on the first layer of Ysgard, Yggdrasil's roots and branches extend into many other planes, providing yet another way for travelers to get around. No one has mapped the extent of these pathways, but some of the important paths are known. One of the roots reaches to Niflheim on the Gray Waste, and another stretches to Pandemonium, where Loki resides. The branches are no less widespread, even crossing the silvery void of the Astral Plane to reach the very Prime Material Plane. There, it's said, the smallest branch tips touch hundreds of worlds where the Norse gods are revered or remembered. Other branches reach Elysium, the Beastlands, and Limbo, and there are likely to be many more such pathways.

Traveling Yggdrasil's paths requires the traveler to climb through the branches or roots. At each planar crossing, the tree disappears through a portal of shimmering color, like a color pool found on the Astral. Only the vaguest of shapes can be seen through the window. To know for certain what lies on the other side, a traveler must step through. The colored portals are all two-way passages.

Note: Yggdrasil originally appeared in this form in the Planescape AD&D campaign setting from TSR. Its use here is for the purposes of providing context for the campaign only.