Yggdrasil
Yggdrasil and Mount Olympus are the two astral landmarks that are normally encountered by travelers from worlds that worship the Norse and Greek pantheons, respectively, but travelers from other Prime Material planes can encounter them. Both are longstanding conduits from the outer planes to Alternate Prime planes. They were each created by a group of Demi-, Lesser, and Greater Powers and worshipers in the Prime Material plane. Yggdrasil is the "World Ash" that links the "important" outer planes to the Prime Material plane, in the Norse pantheon. It runs from Gladsheim, home of most of the Norse pantheon, to Niflheim, the center layer of the three Glooms of Hades and the dwelling place of the goddess of the same name. Roots and branches of Yggdrasil wind through most of the Prime Planes where these deities are recognized.
The "World Ash" is a solid and permanent conduit that weathers the waxing and waning of faiths in the Prime Material and the fortunes of the gods in the outer planes. The traveler is confronted by a huge tree rising from the mist of the Astral Plane and disappearing far into the distance. The traveler can then climb the tree to the appropriate outer plane, descend to the reachable lower planes, or explore the alternate Primes that the tree touches upon.
At the true terminus (reached after the standard amount of travel time), the tree ends in a color pool similar to that of a fixed portal. The traveler can then pass into the outer plane as if moving into an alternate Prime Material or the Astral plane. Yggdrasil and Mount Olympus are the best-known of the permanent conduits that link the outer planes with Prime planes and with other nonlinear outer planes.
Gladsheim is the topmost point of Yggdrasil. The roots of this mighty tree lie in the second layer of Hades, and its branches touch every alternate Prime Material plane where the Norse pantheon has been or is being worshiped. It is conceivable for a traveler to physically move from the Prime Material through the Astral Plane and either to Niflheim or Asgard, though this journey should take at least 100 days and has never been successfully attempted. The World Ash is the home of many giant stags who browse upon its leaves, and it also contains numerous nests of giant eagles. A giant squirrel named Ratatosk scales the tree continually, conveying threats to the eagle and others from the dragons of Niflheim. The apertures that the Yggdrasil causes in the Prime planes are fixed and limited to those places where the Norse gods are known. For this reason, the gods of those that follow the viking way have a second method of quick planar access; the trembling span of the rainbow bridge, which is called Bifrost. Odin or Heimdall can direct the bridge to connect to any Prime Material plane, where it appears as a rainbow leading up to a cloud. In that cloud is the earthly end of a conduit leading to the entrance of Asgard, between the plains of Ida and Vigrid. The bridge is but 10 yards wide (it can be made wider if Odin or Heimdall wishes). While upon this bridge, all magical abilities of beings and items are lost (except for those of Powers and artifacts). While the bridge can be sent to various places in the alternate material planes, its Gladsheim end is always between the realms of Ida and Vigrid, under the watchful eyes of Himinbjorg? and Heimdall.
The Norns are said to have their well of souls at the very edge of the center of the Plane of Concordant Opposition. They identify this area as an unattainable part of the Yggdrasil, though the behavior of this area is unlike that of any other part of the World Ash conduit.
