Flying units pass through standard mountain hexes at a cost of two Move.
Tunnels may pass through Mountain hexes.
Certain mountain areas, like Mount Mofo have trail areas that may be accessed by non-flying units. However, there is a huge movement penalty for these units in this terrain type. In the case of Mount Mofo, for instance, low-move units like siege engines could take three to four turns just to climb the mountain.summer-updates-–-017
According to the Stupid Meal, flying units may not be attacked by non-fliers in mountain hexes. That information was incomplete for forest, so it's possible that Mountain units might also be able to attack fliers in mountains, and only non-Mountain units (on a trail, for instance) would be prevented.
 High Mountains
 Impassable Mountains
A volcano is a form of mountain capable of spewing out lava.
Eruptions can be caused by advanced Dirtamancy (with assistance from a Thinkamancer and Croakamancer within a link)Erf-b1-p137), and inflict significant damage to any units within the affected hexes. Eruptions can also change the terrain type to Lava lake
Units that are incapable of evacuating the affected terrain are at risk of being instantly killed by the volcano. Any unit that survives the initial destruction will take a second wave of high-powered attacks inflicting a combination of fire, explosive and physical damage.
Kills made by a triggered volcanic eruption provide limited or no experience (assuming that the casters survive); there is nothing remarkable about superweapon attacks.