5th Age Campaigns' home rules
Travel distances in the Underdark
Here’s a rough rule of thumb you can use for estimating Underdark travel times, assuming the travelers have sufficient magic & equipment to handle the terrain.
a) Locate the starting point and destination on a map of the surface Realms. Determine the distance in miles between the two points as the crow flies.
b) If there is a regular trade route between the two points, triple the distance.
c) If not, determine if either point is wholly cut off from the rest of the Underdark.
d) If not cut off, multiply the distance by 10.
e) If cut off, repeat the algorithm twice, but this time calculate the distance between the starting point and the nearest surface exit, the time to travel to the destination from the nearest surface entrance, and add those times to the time it takes to travel across the surface.
f) Once the distance is established, use the movement charts in the FR boxed set for rough terrain.
g) Don’t forget to use the tables in the DMG for determining if the travelers have gotten lost.
The motivation for the multipliers is to account for the difficulty of terrain and for the tendency of Underdark tunnels to weave, twist, and double back. It’s a three-dimensional labyrinth after all…
(ref from Candlekeep.com)
While traveling, random encounters are made every mile 2 hours in the upper Underdark, every 2d2-1 hours in the middle underdark and every 2d3-1 hours in the lower underdark.
When camping, encounter checks are made every 1d6 hours if no Shelter, and every 1d8 hours with a shelter.