Whether you're a devout follower of the Norse pantheon and believe that the shrub is the central axis of all existence or simply a planewalker that doesn't mind climbing to his next location, almost everyone has wandered on or wondered at Yggdrasil. Likewise, the greatest mountain the chaotic side of Celestia has brought countless followers of the Olympian deities to their knees in prayer and everybody else in exhaustion. These natural planar pathways play an important part in many Planescape campaigns, but unlike their watery counterparts (the Oceanus and the Styx), they are not so easy to envision.
As far as I understand, the main bulks of both are almost planes to themselves, having conditions seperate from the planes that they touch. Niether Yggdrasil's trunk nor Olympus' slopes are difficult to imagine - just a really big tree trunk (with gravity pointing toward the trunk) and a really big mountain, respectively. What do the parts that connect to the outer planes look like, however?
Yggdrasil throws out branches and roots to the upper and lower planes of chaos. From Yggdrasil's perspective, there is simply a portal as you go along a particular branch. What do they look like from the outer plane's perspective, though? Do the tree's limbs simply appear out of thin air? Is there a part of a branch hanging between two portals (one to Yggdrasil itself and the other to the Astral) or sticking out of the ground? Even though it is said to grow out of Niflheim, its roots reach out to many planes, including the Outlands. So, do you see a huge trunk growing out of Niflheim, or do you just see a single root? How big is Nidhogg, where does the rooster with his hawk-hat reside, why are the ratatosk such gossip mongers, and what the hell happened to the four deer?
Olympus is a bit easier than the tree, but still somewhat confusing. Reaching from the third layer of the Gray Wastes to its main bulk around Arborea, it looks like a simple mountain when you're walking on it. However, does one see a mountain base on both planes, or does it simply connect to portals in caverns around the Outer Planes while looking like a mountian from its own perspective only? What happens if you fly too far from the slope? Do you end up in the Astral, like when you jump off of Yggdrasil, or do you take a long drop and a sudden stop by Hades' doorstep?
Anyway, I think I've posed enough questions. I'd really like to hear some oppinions on the subject, so if you took the time to read all that, please don't be shy to share.
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It really speaks to me.
At least in the 1e MotP, there were just a bunch of color pools on both paths. Both Olympus and Yggdrasil primarily exist on the Astral Plane (but with gravity), and color pools throughout their mass lead to the planes (Outer and Material) where their respective pantheons are revered. Roots, tunnels, and peaks may poke through to other planes, however.
I think it's more interesting to give it a smoother transition, though. The Tree and the Mountain disappear in the infinite distance, and as you travel on them the terrain slowly changes - imperceptibly so - slowly shifting from blue sky scattered with clouds (or the gray, depressing sky of the Waste) to the bright silver of the Astral. You never see exactly where one plane becomes the other; the change is too slow and subtle for that.