The Underdark is an ancient network of tunnels and underground places allowing long-distance travel beneath the earth. In the Caldero cycle it connects Caldero with Star City, and its heart lies about twenty miles from the city.
| Type | Place |
| Spanish entry | Underdark |
- It is entered through the grottoes under the War Tower; the first landmark of the descent is the “wolf’s salt box”.
- Natural passages inside are called “veins of the earth”.
- Reported inhabitants are giants, drow creatures and, unconfirmed, dwarf kingdoms.
- La Jabalina (“the Javelin”) plans an expedition there.