Alter Aeon Online Help

Keywords are:  'subterranean caverns underground sewer catacomb biomes'

There are several kinds of terrain that indicate subterranean locations:

cave        - natural underground spaces, usually formed by wind or water erosion
underground - large underground tunnels or chambers, far from the surface. Also
              includes lava tubes and volcanic formations
lava        - rarely found on the surface, magma, masses and pools of molten rock
catacomb    - tunnels and chambers created by sentient beings, often as part of
              funerary complexes
sewer       - underground tunnels used for managing waste water, most often found
              beneath very large cities

Subterranean terrains tend to be dark. There is little to forage and no wood, and
herbs are scarce. They tend to be rich in mineral resources, and certain rare
minerals can only be found near lava.

The 'climbing' skill may help you move about subterranean spaces more safely, and
the 'spelunking' skill lowers the movement cost when in moving natural caves.