It is an issue that has been discussed several times, but I'm bringing it again so we can pool the new information.
Kern has said that the Mists help keep mana in the Underworld rather than disperse quickly in the atmosphere.
It seems that mana gets blocked by solid objects, so it disperses faster on the surface than in caves, on the surface than in the Mists, and on the plains than in forests.
It seems there are isolated mana-rich zones withing the Nidavellir.
Diva says that the World Tree of Mimaneid feeds from the Mists of the Nidavellir, and in turn produces mana that spreads to the Underworld.
It seems that the airships can get mana from the Mists. Kern hints at it.
In Path to Power, it's pretty much confirmed that the ships get energy from the Mists. And not just the Mists around a mana pool, but from those in the middle of the Nidavellir.
This page explicitly says that the drow's machines are "powered by the Mists".
We have seen one of those machines, the airship. My question is, what about other machines? Do any of you found any reference to any other machine able to drain mana from the Mists?