For the Player's Handbook Ranger (not Unearthed Arcana Revised Ranger), does the Natural Explorer terrain type of "Underdark" really mean any underground terrain, such as an underground cave? Of the listed terrain types (Arctic, Coast, Desert, Forest, Grassland, Mountain, Swamp, Underdark) there isn't really anything else that an underground cave would fall under, if not "Underdark."


1 Answer 1


It depends on the setting.

In some Dnd settings the Underdark is more than just a cave or an underground passage. It's a massive underground network that can potentially span an entire continent. It's kind of like another layer of the world.

In other settings this layer may not exist, or be a lot smaller in scope. In this case it's up to your DM to decide what defines Underdark. It could be any underground area, an especially deep/dark underground area or a specific layer as above.

In my current homebrew campaign setting I just define it as any large natural underground area. This brings it more inline with the other terrain types (Arctic, Coast, Desert, Forest, Grassland, Mountain, Swamp) and also means it is more likely to be useful for the ranger, as the party journeying to the Underdark proper is unlikely.

  • \$\begingroup\$ So does this mean that in settings such as the FR where the Underdark is well defined, a mere cave (such as Wave Echo Cave from the Lost of Mines of Phandelver published adventure) would not fall under any of the Natural Explorer terrain types? \$\endgroup\$
    – mdrichey
    Sep 4, 2018 at 5:02
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    \$\begingroup\$ They are not counted as Underdark. I believe there is a even a passage describing a tunnel in Wave Echo Cave that specifically states it leads to the Underdark ie. Wave Echo Cave is not part of the Underdark. The cave is in the mountains though so I'd rule the terrain counts as Mountain in that example. My understanding is that everywhere (atleast on the material plane) has a terrain type. \$\endgroup\$
    – RedTera
    Sep 4, 2018 at 5:12
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    \$\begingroup\$ @RedTera: The Underdark is not mentioned anywhere in LMOP. \$\endgroup\$
    – V2Blast
    Sep 4, 2018 at 5:43
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    \$\begingroup\$ @RedTera I think I will need to ask a separate question to confirm whether it is correct that everywhere on the material plane has a terrain type, and that the correct terrain type for Wave Echo Cave is Mountain. \$\endgroup\$
    – mdrichey
    Sep 6, 2018 at 21:43
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    \$\begingroup\$ @RedTera followup question asked here: link \$\endgroup\$
    – mdrichey
    Sep 6, 2018 at 22:03

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