[The Amber Roads play a critical role for both svirfneblin and zwerc; for more information about the latter get your hands on the Underworld Races PDF, and for more information on both keep your eyes peeled! -MM]
These veins of strange translucent orange crystal are permeable to living creatures, changing how quickly and safely they can travel through Aventyr’s Underworld. What more these pathways are planar, and certain branches lead directly to pockets of demiplane connected to the Elemental Plane of Earth. To the uninitiated and unwary, they can be deadly, but for those who know the Amber Roads, there’s no better means to journey. Finding an entryway to the Amber Roads requires a Knowledge (geography) check that varies in difficulty determined by region (at the GM’s discretion, but typically DC 21).
Amber Roads have the following traits (otherwise they operate as the Material Plane does):
Restricted Spaces: The vast majority of passages that make up the Amber Roads are rarely taller or wider than 10 feet across (90% of it is composed of 10-ft.-radius tunnels).
Heavy Gravity: Acrobatics, Climb, Ride, and Swim checks incur a –2 circumstance penalty, as do all attack rolls. All item weights are effectively doubled, which might affect a character's speed. Weapon ranges are halved. A character's Strength and Dexterity scores are not affected.
Finite Size*: The Amber Roads are thought to have limitations, though efforts to map their entirety are not yet complete.
Enhanced Earth Magic: Spells and spell-like abilities with the earth descriptor are easier to use or more powerful in effect on Amber Roads than they are on the Material Plane. Zwerc and svirfneblin are aware of which spells and spell-like abilities are enhanced, but planar travelers may have to discover this on their own. If a spell is enhanced, it functions as if its caster level was 2 higher than normal.
Fragile: The Amber Roads have suffered greatly from the doings of the svirfneblin, making them sometimes unreliable. There is a 5% chance every hour that an Amber Road tunnel suffers a cave-in.
Overland Movement: While traveling on Amber Roads, the overland movement speed of a creature can be impeded or aided depending on its size. The speed of Large-sized creature is halved and the speed of Medium-sized creatures is doubled. Dwarves (all subtypes) and Small-sized creatures triple their normal overland movement rate when traveling on Amber Roads. Huge-sized and larger creatures lacking the compression ability (or a similar ability that allows travel in the confined passages) are unable to enter or travel through the vast majority of Amber Roads. Creatures with the Earth subtype always have their overland movement speed tripled and Amber Roads expand to accommodate their size.
Refractions: Non-dwarves, non-gnomes, and creatures lacking the earth subtype are prone to quickly losing their way in the junctions and passages of the Amber Roads. A Knowledge (geography) or Survival check (DC 15 + 1 per previous check) must be made every 8 hours to properly navigate the planar pathway to avoid becoming lost. Parties that are not tethered together somehow (via rope, chains, or other durable material) cannot share a collective navigation check or aid one another.
*The Amber Roads do have hard boundaries, but they are specifically not mapped out to better provide a plot device for GMs to use that allows for faster travel throughout the Underworld when appropriate.