Subterranean/Underland Movement Speeds?

[Ogrork the Mighty]

Is there a movement rate for travelling underground through subterranean passages/caves. I'm looking for an average hourly or daily movement rate as opposed to tactical rates. The only thing I can come up with is "difficult terrain" which has x2 movement cost. The party will be traveling for long periods underground and I need to figure out how far they can move in an hour and per day.

Maybe in the FR Underdark book?

 

[RedShirtNo5]

I was looking for the same thing just a few days ago, and didn't find anything specific. It's not in FR Underdark. Also, it depends on your conception of the underdark; what sort of events or processes create the caves and tunnels down there? Are these "realistic" caves, or your stereotypical fantasy underground? Anyway, based on the x2 for difficult terrain and a comparison to mountainous terrain in PHB, I decided on the following:

higways: x1 = passages formed by processes (e.g., burrowing monsters, particularly delvers) that leave smooth, straight tunnels
primary passages: x3/4 = natural passages formed by processes that leave ledges, stalagmites, etc., but which are well travelled and have been extensively worked
secondary passages: x1/2 = natural passages formed by processes that leave ledges, stalagmites, etc., but are generally navigable [my assumption being that a default fantasy undergound is navigable but "difficult terrain"]
difficult passages: x1/4 = very difficult cave area, e.g., limestone caves with many changes in elevation, the sort of thing spelunkers/cavers tackle IRL

chasms, cliffs, pits, chimneys, and such need to be handled individually until nearly everyone has fly

FYI, FR Underdark does have a recommended wandering monster frequency:
desolate: 1 in 20
wilderness: 1 in 12
frontier: 1 in 10
settled: 1 in 8
check per hour of normal travel, 1/2 chance if camping or cautious travel, 1/4 chance if concealed camping

-RedShirt