The mushroom forest in the subterranean regions has a very strange aspect. At first Axel takes the mushrooms for tall and peculiarly motionless, colourless trees. Here, however, the forests contain not trees, as on the surface of the earth, but various growths that on earth normally remain small and underfoot. Due to the different conditions inside the earth, however, such as higher levels of heat and moisture, the mushrooms and various 'lowly shrubs' have sprouted to massive heights. Therefore they are weird and unnatural in appearance.
Axel feels rather uneasy while in the mushroom forest:
...it was mortally cold under those fleshy vaults. For half an hour we wandered about those damp shadows, and it was with a feeling of relief that I regained the sea-shore. (chapter 29)
The mushroom forest, then, appears as a somewhat unsettling place. Even if there is a perfectly logical, scientific explanation for these forests inside the earth, Axel finds it very strange to be among them.