This is a reference to permafrost, which is soil at or below the freezing point of water.
The picture shows two men trying to dig in permafrost and having to resort to jackhammers.
In Lyra's world a fire-mine is a geo-thermal vent.
Every character in Northern Lights is at least vaguely aware of what a cliff-ghast is, and many of them – such as Serafina, Iorek and Lee – have encountered and fought them. Consequently, we as readers are never given an clear explanation.
Unlike night-ghasts (the only other kind of ghasts mentioned in the novel), cliff-ghasts appear to be mortal. They are scavengers of sorts, living off the witches and bears like vultures live off lions. They are described as being winged and having the appearance of something like a goblin.