GURPSよろず

「物質透過」 accounts for the traditional ability of otherwise-tangible nature spirits to pass through mountains, trees, and so on with ease.
Supers with elemental powers often possess similar capabilities --as do fiendish fantasy monsters that step out of dungeon walls to eat adventurers!
「物質透過」 is normally associated with a particular class of materials. If the GM wishes, though, he can permit a further class --“Everything” --that lets the user walk through any solid substance. This costs 80 points.

■Alternatives
Those who have 「負傷耐性」 (「[集合体?」) (p. 52) modified with Infiltration can seep through porous obstacles. Users of 「非実体化」 (p. 55) are intangible, and can pass through anything. To burrow through barriers physically, take 「トンネル移動」 (p. 85).

■New Special Enhancements
Extended: You can pass through multiple, similar materials. Buy the most expensive one at full price.
Each additional, related substance is +20%. For instance, someone with 「物質透過」 (Stone) could add Extended, Earth and Metal for +40%.

Meld 1: You can become one with the things you permeate, like a dryad merging with her tree. This works only on discrete objects --a tree (not a forest), a mountain (not a planet), etc. Once melded, you can use your senses from or step out of the object at any point along its interior or exterior surfaces, and can breathe if any part of it is touching air. Take a Ready maneuver to shift viewpoints or exit the object. If the object takes damage, it affects you proportionally; e.g., 10 HP of injury to a 100 HP tree results in a 1 HP wound for a 10 HP man. Being part of something larger than you also blunts 「特殊効果」s: add the difference between its SM and yours to your resistance roll. If you have attacks that affect those touching or standing near you, like 「放射」s, they’re “spread thin” if you merge with a large object: scale down damage in proportion to HP (to 1/10 normal, in the above example), and add the difference between the object’s SM and your own to rolls to resist your 「特殊効果」s. You can’t control the items you meld with; get 「憑依」 for that. +150%.

Meld 2: As Meld 1, but works even on distributed objects. With 「物質透過」 (Earth), you could meld with the ground at your feet to cross the entire planet in the blink of an eye; with 「物質透過」 (Wood), you could meld with a forest to traverse it instantly or spy on events within. +300%.

■New Special Limitation
Finite Thickness: You can only pass through a barrier of limited thickness.
If you try to move through anything thicker, you penetrate as far as your limit and then run into a solid obstacle.

Value depends on thickness: -50% for a thin membrane (like a curtain), -30% for an inch, -15% for a foot, and -5% for the distance you can sprint in a second.

■Powering Up
「物質透過」 is a typical feature of elemental and nature-spirit powers. It also suits divine powers bestowed by deities associated with the substances it affects. 才能 adds to all DX or skill rolls made for movement while using this ability.