In the 3.5 Players Handbook, the spell "Rope Trick" allows a mage to create a temporary pocket plane which 8 medium humanoids (or an equivalent mass) can hide within. The point of entry to this plane is attached to a length of rope, which must be climbed in order to enter this portal.
The spell description states that extreme levels of force can rip this rope free - which would cause exiting creatures to fall out of the portal once they choose to leave, or when the spell ends. In this instance the planar pocket remains in its default casting location.
Nowhere in the spell description does the text state that this pocket plane is anchored to anything other than the rope that connects to the plane's portal. Conversely (in more conventional parts of the Prime anyway), the portal already has to constantly move through space in order to compensate for the Prime World's orbital path and daily rotation cycle. As a default assumption, it would seem that the caster chooses for the bottom end of the rope to be conceptually linked to the spatial coordinates of the surface (or equivalent) that she casts the spell from - and the magic causes it to rise up from here.
Since the spell automatically compensates for any planetary motion (on applicable worlds) - then presumably this would also apply to large moving Extraplanar masses, such as Acheron Cubes and Mechanus Cogs.
But how far should this realistically stretch?
1. If a wizard cast a rope trick on a moving ship, then would the rope rapidly disappear over the stern to hang in the air over the ocean? Or would it keep pace with the ship as well?
2. Since this conceptual linking to the immediate coordinates is never actually stated, could the caster choose not to do this and instead just create an non- spatially attached pocket plane joined to one end of a vertically aligned rope? You could do this in very static places like the Ethereal Plane, the Negative Energy Plane, and certain Outer Planes such as Baator presumably. Some Prime Worlds are stationary in space too.
3. We know that the portal cannot be physically pulled through space, as the rope is pulled free by more than 16,000 pounds of force. If the rope is either pulled free, or drawn into the portal, then anyone in the rope trick is assumed to remain fixed in 3D space at the site of the spell's casting (see above in terms of automatic compensatory spatial adjustment)
What would occur if someone cast "Teleport" on the rope? No physical forces would have been applied, yet the rope's physical location would suddenly change.
Would this move the entrance to the Rope Trick as well? ( remember that unless the rope is "physically" moved, then its top end is the default location of the pocket plane's entrance).
Alternatively, would the rope just disappear, and the planar pocket still remain in exactly the same place?
Why? Why not?
I play with philosophers and they can be really annoying.
Conceptually if think of the spell as this, if it only affected 3 dimensions it would create a pit in the ground or large moving object it was on that could fit a bunch of people. The pit wouldn't move anywhere as it would move with it. Rope Trick effectively creates a pit, except that it's in something that's beyond 3 dimensional space.